Adds implementation for relation-departed hooks to cleanly remove
participant sites from the multisite system. The replication
between zones is stopped and both zones split up to continue as
separate master zones.
Change-Id: I420f7933db55f3004f752949b5c09b1b79774f64
func-test-pr: https://github.com/openstack-charmers/zaza-openstack-tests/pull/863