Found this snippet in the cloud-init.log of the failed node:

Jan  4 22:19:59 azurill [CLOUDINIT] handlers.py[DEBUG]: start: modules-final/config-scripts-user: running config-scripts-user with frequency once-per-instance
Jan  4 22:19:59 azurill [CLOUDINIT] url_helper.py[DEBUG]: [0/1] open 'http://10.245.208.27:5240/MAAS/metadata/status/cxcn4e' with {'allow_redirects': True, 'headers': {'Authorization': 'OAuth oauth_nonce="55078351223699034471483568399", oauth_timestamp="1483568399", oauth_version="1.0", oauth_signature_method="PLAINTEXT", oauth_consumer_key="Gw7h5Ta7eGVg37f89f", oauth_token="Ucmx8eJQWDqxankmxt", oauth_signature="%26dgZVCxfXGZFzWrTNQycqzYEFuxRuFS6D"'}, 'method': 'POST', 'url': 'http://10.245.208.27:5240/MAAS/metadata/status/cxcn4e'} configuration
Jan  4 22:19:59 azurill [CLOUDINIT] url_helper.py[DEBUG]: Read from http://10.245.208.27:5240/MAAS/metadata/status/cxcn4e (200, 2b) after 1 attempts
Jan  4 22:19:59 azurill [CLOUDINIT] util.py[DEBUG]: Writing to /var/lib/cloud/instances/cxcn4e/sem/config_scripts_user - wb: [420] 24 bytes
Jan  4 22:19:59 azurill [CLOUDINIT] helpers.py[DEBUG]: Running config-scripts-user using lock (<FileLock using file '/var/lib/cloud/instances/cxcn4e/sem/config_scripts_user'>)
Jan  4 22:19:59 azurill [CLOUDINIT] util.py[DEBUG]: Running command ['/var/lib/cloud/instance/scripts/part-001'] with allowed return codes [0] (shell=False, capture=False)
Jan  4 22:20:01 azurill [CLOUDINIT] util.py[WARNING]: Failed running /var/lib/cloud/instance/scripts/part-001 [3]
Jan  4 22:20:01 azurill [CLOUDINIT] util.py[DEBUG]: Failed running /var/lib/cloud/instance/scripts/part-001 [3]#012Traceback (most recent call last):#012  File "/usr/lib/python3/dist-packages/cloudinit/util.py", line 793, in runparts#012    subp(prefix + [exe_path], capture=False)#012  File "/usr/lib/python3/dist-packages/cloudinit/util.py", line 1836, in subp#012    cmd=args)#012cloudinit.util.ProcessExecutionError: Unexpected error while running command.#012Command: ['/var/lib/cloud/instance/scripts/part-001']#012Exit code: 3#012Reason: -#012Stdout: ''#012Stderr: ''
Jan  4 22:20:01 azurill [CLOUDINIT] cc_scripts_user.py[WARNING]: Failed to run module scripts-user (scripts in /var/lib/cloud/instance/scripts)
Jan  4 22:20:01 azurill [CLOUDINIT] handlers.py[DEBUG]: finish: modules-final/config-scripts-user: FAIL: running config-scripts-user with frequency once-per-instance
Jan  4 22:20:01 azurill [CLOUDINIT] url_helper.py[DEBUG]: [0/1] open 'http://10.245.208.27:5240/MAAS/metadata/status/cxcn4e' with {'allow_redirects': True, 'headers': {'Authorization': 'OAuth oauth_nonce="69625729650797160551483568401", oauth_timestamp="1483568401", oauth_version="1.0", oauth_signature_method="PLAINTEXT", oauth_consumer_key="Gw7h5Ta7eGVg37f89f", oauth_token="Ucmx8eJQWDqxankmxt", oauth_signature="%26dgZVCxfXGZFzWrTNQycqzYEFuxRuFS6D"'}, 'method': 'POST', 'url': 'http://10.245.208.27:5240/MAAS/metadata/status/cxcn4e'} configuration
Jan  4 22:20:01 azurill [CLOUDINIT] url_helper.py[DEBUG]: Read from http://10.245.208.27:5240/MAAS/metadata/status/cxcn4e (200, 2b) after 1 attempts
Jan  4 22:20:01 azurill [CLOUDINIT] util.py[WARNING]: Running module scripts-user (<module 'cloudinit.config.cc_scripts_user' from '/usr/lib/python3/dist-packages/cloudinit/config/cc_scripts_user.py'>) failed
Jan  4 22:20:01 azurill [CLOUDINIT] util.py[DEBUG]: Running module scripts-user (<module 'cloudinit.config.cc_scripts_user' from '/usr/lib/python3/dist-packages/cloudinit/config/cc_scripts_user.py'>) failed#012Traceback (most recent call last):#012  File "/usr/lib/python3/dist-packages/cloudinit/stages.py", line 792, in _run_modules#012    freq=freq)#012  File "/usr/lib/python3/dist-packages/cloudinit/cloud.py", line 70, in run#012    return self._runners.run(name, functor, args, freq, clear_on_fail)#012  File "/usr/lib/python3/dist-packages/cloudinit/helpers.py", line 200, in run#012    return (True, results)#012  File "/usr/lib/python3.5/contextlib.py", line 77, in __exit__#012    self.gen.throw(type, value, traceback)#012  File "/usr/lib/python3/dist-packages/cloudinit/helpers.py", line 88, in lock#012    yield self._acquire(name, freq)#012  File "/usr/lib/python3/dist-packages/cloudinit/helpers.py", line 199, in run#012    results = functor(*args)#012  File "/usr/lib/python3/dist-packages/cloudinit/config/cc_scripts_user.py", line 57, in handle#012    util.runparts(runparts_path)#012  File "/usr/lib/python3/dist-packages/cloudinit/util.py", line 800, in runparts#012    % (len(failed), len(attempted)))#012RuntimeError: Runparts: 1 failures in 1 attempted commands
Jan  4 22:20:01 azurill [CLOUDINIT] stages.py[DEBUG]: Running module ssh-authkey-fingerprints (<module 'cloudinit.config.cc_ssh_authkey_fingerprints' from '/usr/lib/python3/dist-packages/cloudinit/config/cc_ssh_authkey_fingerprints.py'>) with frequency once-per-instance
Jan  4 22:20:01 azurill [CLOUDINIT] handlers.py[DEBUG]: start: modules-final/config-ssh-authkey-fingerprints: running config-ssh-authkey-fingerprints with frequency once-per-instance
Jan  4 22:20:01 azurill [CLOUDINIT] url_helper.py[DEBUG]: [0/1] open 'http://10.245.208.27:5240/MAAS/metadata/status/cxcn4e' with {'allow_redirects': True, 'headers': {'Authorization': 'OAuth oauth_nonce="110389373849726595161483568401", oauth_timestamp="1483568401", oauth_version="1.0", oauth_signature_method="PLAINTEXT", oauth_consumer_key="Gw7h5Ta7eGVg37f89f", oauth_token="Ucmx8eJQWDqxankmxt", oauth_signature="%26dgZVCxfXGZFzWrTNQycqzYEFuxRuFS6D"'}, 'method': 'POST', 'url': 'http://10.245.208.27:5240/MAAS/metadata/status/cxcn4e'} configuration
Jan  4 22:20:01 azurill [CLOUDINIT] url_helper.py[DEBUG]: Read from http://10.245.208.27:5240/MAAS/metadata/status/cxcn4e (200, 2b) after 1 attempts
Jan  4 22:20:01 azurill [CLOUDINIT] util.py[DEBUG]: Writing to /var/lib/cloud/instances/cxcn4e/sem/config_ssh_authkey_fingerprints - wb: [420] 24 bytes
Jan  4 22:20:01 azurill [CLOUDINIT] helpers.py[DEBUG]: Running config-ssh-authkey-fingerprints using lock (<FileLock using file '/var/lib/cloud/instances/cxcn4e/sem/config_ssh_authkey_fingerprints'>)
Jan  4 22:20:01 azurill [CLOUDINIT] util.py[DEBUG]: Reading from /etc/ssh/sshd_config (quiet=False)
Jan  4 22:20:01 azurill [CLOUDINIT] util.py[DEBUG]: Read 2540 bytes from /etc/ssh/sshd_config
Jan  4 22:20:01 azurill [CLOUDINIT] util.py[DEBUG]: Reading from /home/ubuntu/.ssh/authorized_keys (quiet=False)
Jan  4 22:20:01 azurill [CLOUDINIT] util.py[DEBUG]: Read 397 bytes from /home/ubuntu/.ssh/authorized_keys
Jan  4 22:20:02 azurill [CLOUDINIT] handlers.py[DEBUG]: finish: modules-final/config-ssh-authkey-fingerprints: SUCCESS: config-ssh-authkey-fingerprints ran successfully
Jan  4 22:20:02 azurill [CLOUDINIT] url_helper.py[DEBUG]: [0/1] open 'http://10.245.208.27:5240/MAAS/metadata/status/cxcn4e' with {'allow_redirects': True, 'headers': {'Authorization': 'OAuth oauth_nonce="120427651392587778991483568402", oauth_timestamp="1483568402", oauth_version="1.0", oauth_signature_method="PLAINTEXT", oauth_consumer_key="Gw7h5Ta7eGVg37f89f", oauth_token="Ucmx8eJQWDqxankmxt", oauth_signature="%26dgZVCxfXGZFzWrTNQycqzYEFuxRuFS6D"'}, 'method': 'POST', 'url': 'http://10.245.208.27:5240/MAAS/metadata/status/cxcn4e'} configuration
Jan  4 22:20:02 azurill [CLOUDINIT] url_helper.py[DEBUG]: Read from http://10.245.208.27:5240/MAAS/metadata/status/cxcn4e (200, 2b) after 1 attempts
Jan  4 22:20:02 azurill [CLOUDINIT] stages.py[DEBUG]: Running module keys-to-console (<module 'cloudinit.config.cc_keys_to_console' from '/usr/lib/python3/dist-packages/cloudinit/config/cc_keys_to_console.py'>) with frequency once-per-instance