trusty-mitaka: hook failed: "cloud-compute-relation-changed" for nova-compute:cloud-compute

In the unit logs for nova-cloud-controller:

018-03-06 17:24:19 DEBUG juju-log cloud-compute:19: OpenStack release, database, or rabbitmq not ready for Cells V2
2018-03-06 17:24:20 DEBUG cloud-compute-relation-changed getaddrinfo juju-875500-icey-12: Name or service not known
2018-03-06 17:24:20 ERROR juju-log cloud-compute:19: Could not obtain SSH host key from juju-875500-icey-12
2018-03-06 17:24:20 DEBUG cloud-compute-relation-changed Traceback (most recent call last):
2018-03-06 17:24:20 DEBUG cloud-compute-relation-changed File "/var/lib/juju/agents/unit-nova-cloud-controller-0/charm/hooks/cloud-compute-relation-changed", line 1150, in <module>
2018-03-06 17:24:20 DEBUG cloud-compute-relation-changed main()
2018-03-06 17:24:20 DEBUG cloud-compute-relation-changed File "/var/lib/juju/agents/unit-nova-cloud-controller-0/charm/hooks/cloud-compute-relation-changed", line 1144, in main
2018-03-06 17:24:20 DEBUG cloud-compute-relation-changed hooks.execute(sys.argv)
2018-03-06 17:24:20 DEBUG cloud-compute-relation-changed File "/var/lib/juju/agents/unit-nova-cloud-controller-0/charm/hooks/charmhelpers/core/hookenv.py", line 800, in execute
2018-03-06 17:24:20 DEBUG cloud-compute-relation-changed self._hooks[hook_name]()
2018-03-06 17:24:20 DEBUG cloud-compute-relation-changed File "/var/lib/juju/agents/unit-nova-cloud-controller-0/charm/hooks/cloud-compute-relation-changed", line 663, in compute_changed
2018-03-06 17:24:20 DEBUG cloud-compute-relation-changed ssh_compute_add(key, rid=rid, unit=unit)
2018-03-06 17:24:20 DEBUG cloud-compute-relation-changed File "/var/lib/juju/agents/unit-nova-cloud-controller-0/charm/hooks/nova_cc_utils.py", line 1005, in ssh_compute_add
2018-03-06 17:24:20 DEBUG cloud-compute-relation-changed add_known_host(host, unit, user)
2018-03-06 17:24:20 DEBUG cloud-compute-relation-changed File "/var/lib/juju/agents/unit-nova-cloud-controller-0/charm/hooks/nova_cc_utils.py", line 955, in add_known_host
2018-03-06 17:24:20 DEBUG cloud-compute-relation-changed raise e
2018-03-06 17:24:20 DEBUG cloud-compute-relation-changed subprocess.CalledProcessError: Command '['ssh-keyscan', '-H', '-t', 'rsa', u'juju-875500-icey-12']' returned non-zero exit status 255
2018-03-06 17:24:20 ERROR juju.worker.uniter.operation runhook.go:107 hook "cloud-compute-relation-changed" failed: exit status 1