swift-storage-relation-changed: No such file or directory: '/etc/swift/account.builder'

Landscape autopilot deployments of Openstack Newton in CI have hit two cases of the following hook error:

[from landscape-0-inner-logs/swift-proxy-2/var/log/juju/unit-swift-proxy-2.log]
2017-01-23 07:56:33 INFO swift-storage-relation-changed Traceback (most recent call last):
2017-01-23 07:56:33 INFO swift-storage-relation-changed File "/var/lib/juju/agents/unit-swift-proxy-2/charm/hooks/
swift-storage-relation-changed", line 734, in <module>
2017-01-23 07:56:33 INFO swift-storage-relation-changed main()
2017-01-23 07:56:33 INFO swift-storage-relation-changed File "/var/lib/juju/agents/unit-swift-proxy-2/charm/hooks/swift-storage-relation-changed", line 727, in main
2017-01-23 07:56:33 INFO swift-storage-relation-changed hooks.execute(sys.argv)
2017-01-23 07:56:33 INFO swift-storage-relation-changed File "/var/lib/juju/agents/unit-swift-proxy-2/charm/hooks/charmhelpers/core/hookenv.py", line 715, in execute
2017-01-23 07:56:33 INFO swift-storage-relation-changed self._hooks[hook_name]()
2017-01-23 07:56:33 INFO swift-storage-relation-changed File "/var/lib/juju/agents/unit-swift-proxy-2/charm/hooks/charmhelpers/contrib/openstack/utils.py", line 1817, in wrapped_f
2017-01-23 07:56:33 INFO swift-storage-relation-changed restart_functions)
2017-01-23 07:56:33 INFO swift-storage-relation-changed File "/var/lib/juju/agents/unit-swift-proxy-2/charm/hooks/charmhelpers/core/host.py", line 524, in restart_on_change_helper
2017-01-23 07:56:33 INFO swift-storage-relation-changed r = lambda_f()
2017-01-23 07:56:33 INFO swift-storage-relation-changed File "/var/lib/juju/agents/unit-swift-proxy-2/charm/hooks/charmhelpers/contrib/openstack/utils.py", line 1816, in <lambda>
2017-01-23 07:56:33 INFO swift-storage-relation-changed (lambda: f(*args, **kwargs)), restart_map, stopstart,
2017-01-23 07:56:33 INFO swift-storage-relation-changed File "/var/lib/juju/agents/unit-swift-proxy-2/charm/hooks/swift-storage-relation-changed", line 298, in storage_changed
2017-01-23 07:56:33 INFO swift-storage-relation-changed zone = get_zone(config('zone-assignment'))
2017-01-23 07:56:33 INFO swift-storage-relation-changed File "/var/lib/juju/agents/unit-swift-proxy-2/charm/hooks/lib/swift_utils.py", line 591, in get_zone
2017-01-23 07:56:33 INFO swift-storage-relation-changed builder = _load_builder(ring)
2017-01-23 07:56:33 INFO swift-storage-relation-changed File "/var/lib/juju/agents/unit-swift-proxy-2/charm/hooks/lib/swift_utils.py", line 460, in _load_builder
2017-01-23 07:56:33 INFO swift-storage-relation-changed builder = pickle.load(open(path, 'rb'))
2017-01-23 07:56:33 INFO swift-storage-relation-changed IOError: [Errno 2] No such file or directory: '/etc/swift/account.builder'
2017-01-23 07:56:33 ERROR juju.worker.uniter.operation runhook.go:107 hook "swift-storage-relation-changed" failed: exit status 1

This looks very similar to lp:1510865, but the hook here is '-changed' instead of '-joined'.

Landscape version: 16.11~bzr10838+jenkins3390-2
Juju version: 1:2.1~beta4-0ubuntu1~16.04.1~juju1
Charm version: cs:xenial/swift-proxy-57

CI builds:
 - https://ci.lscape.net/job/landscape-system-tests/5073
 - https://ci.lscape.net/job/landscape-system-tests/4979