Fix up failover_host exceptions to preserve states

Fix up failover_host exceptions to preserve states

UnableToFailOver and InvalidReplicationTarget do not save the state
of replication_status currently. This patch adds host.save() in
order to write the change to the db.

In addition, the manager should honor the current replication state
of the host when InvalidReplicationTarget is raised instead of forcing
it into 'enabled' state.

Change-Id: I01af6d70539e518936c74e425c3fad60f4b94581