A recent change to charms.openstack causes a cluster adapter to be
created if it is missing which invokes a call to
reactive.RelationBase.from_state('cluster.connected') which fails
outside of a juju env. This unit test change passes a cluster
relation to the adapter class to stop one being automatically
created.
Closes-Bug: 1630358
Change-Id: Ifb23e39b9ee94620705688841fd11db641884f74