ceph-radosgw fails to upgrade to source:trusty-mitaka

I've attempted to upgrade the ceph-radosgw from source:trusty-liberty to source:trusty-mitaka

ubuntu@maascontroller:~$ juju upgrade-charm ceph-radosgw
Added charm "cs:trusty/ceph-radosgw-19" to the environment.
ubuntu@maascontroller:~$ juju set ceph-radosgw source=cloud:trusty-mitaka

on the actual units:

2016-01-29 01:26:53 INFO upgrade-charm Traceback (most recent call last):
2016-01-29 01:26:53 INFO upgrade-charm File "/var/lib/juju/agents/unit-ceph-radosgw-0/charm/hooks/upgrade-charm", line 446, in <module>
2016-01-29 01:26:53 INFO upgrade-charm hooks.execute(sys.argv)
2016-01-29 01:26:53 INFO upgrade-charm File "/var/lib/juju/agents/unit-ceph-radosgw-0/charm/hooks/charmhelpers/core/hookenv.py", line 717, in execute
2016-01-29 01:26:53 INFO upgrade-charm self._hooks[hook_name]()
2016-01-29 01:26:53 INFO upgrade-charm File "/var/lib/juju/agents/unit-ceph-radosgw-0/charm/hooks/charmhelpers/core/host.py", line 397, in wrapped_f
2016-01-29 01:26:53 INFO upgrade-charm f(*args, **kwargs)
2016-01-29 01:26:53 INFO upgrade-charm File "/var/lib/juju/agents/unit-ceph-radosgw-0/charm/hooks/upgrade-charm", line 297, in config_changed
2016-01-29 01:26:53 INFO upgrade-charm identity_changed(relid=r_id)
2016-01-29 01:26:53 INFO upgrade-charm File "/var/lib/juju/agents/unit-ceph-radosgw-0/charm/hooks/charmhelpers/core/host.py", line 397, in wrapped_f
2016-01-29 01:26:53 INFO upgrade-charm f(*args, **kwargs)
2016-01-29 01:26:53 INFO upgrade-charm File "/var/lib/juju/agents/unit-ceph-radosgw-0/charm/hooks/upgrade-charm", line 366, in identity_changed
2016-01-29 01:26:53 INFO upgrade-charm identity_joined(relid)
2016-01-29 01:26:53 INFO upgrade-charm File "/var/lib/juju/agents/unit-ceph-radosgw-0/charm/hooks/upgrade-charm", line 358, in identity_joined
2016-01-29 01:26:53 INFO upgrade-charm setup_keystone_certs(unit=unit, rid=relid)
2016-01-29 01:26:53 INFO upgrade-charm File "/var/lib/juju/agents/unit-ceph-radosgw-0/charm/hooks/upgrade-charm", line 192, in setup_keystone_certs
2016-01-29 01:26:53 INFO upgrade-charm from keystoneclient.exceptions import ConnectionRefused
2016-01-29 01:26:53 INFO upgrade-charm File "/usr/lib/python2.7/dist-packages/keystoneclient/__init__.py", line 35, in <module>
2016-01-29 01:26:53 INFO upgrade-charm __version__ = pbr.version.VersionInfo('python-keystoneclient').version_string()
2016-01-29 01:26:53 INFO upgrade-charm File "/usr/lib/python2.7/dist-packages/pbr/version.py", line 457, in version_string
2016-01-29 01:26:53 INFO upgrade-charm return self.semantic_version().brief_string()
2016-01-29 01:26:53 INFO upgrade-charm File "/usr/lib/python2.7/dist-packages/pbr/version.py", line 452, in semantic_version
2016-01-29 01:26:53 INFO upgrade-charm self._semantic = self._get_version_from_pkg_resources()
2016-01-29 01:26:53 INFO upgrade-charm File "/usr/lib/python2.7/dist-packages/pbr/version.py", line 432, in _get_version_from_pkg_resources
2016-01-29 01:26:53 INFO upgrade-charm provider = pkg_resources.get_provider(requirement)
2016-01-29 01:26:53 INFO upgrade-charm File "/usr/share/python-wheels/setuptools-3.3-py2.py3-none-any.whl/pkg_resources.py", line 223, in get_provider
2016-01-29 01:26:53 INFO upgrade-charm return working_set.find(moduleOrReq) or require(str(moduleOrReq))[0]
2016-01-29 01:26:53 INFO upgrade-charm File "/usr/share/python-wheels/setuptools-3.3-py2.py3-none-any.whl/pkg_resources.py", line 725, in require
2016-01-29 01:26:53 INFO upgrade-charm needed = self.resolve(parse_requirements(requirements))
2016-01-29 01:26:53 INFO upgrade-charm File "/usr/share/python-wheels/setuptools-3.3-py2.py3-none-any.whl/pkg_resources.py", line 632, in resolve
2016-01-29 01:26:53 INFO upgrade-charm raise VersionConflict(dist,req) # XXX put more info here
2016-01-29 01:26:53 INFO upgrade-charm pkg_resources.VersionConflict: (six 1.5.2 (/usr/lib/python2.7/dist-packages), Requirement.parse('six>=1.9.0'))
2016-01-29 01:26:53 ERROR juju.worker.uniter.operation runhook.go:107 hook "upgrade-charm" failed: exit status 1