Update charm to support Ceilometer@Queens; specifically:

 - Removal of Ceilometer API service
 - Removal of ceilometer-collector service

Most of this change is related to dropping of these
services; In addition its now possible to complete a
ceilometer-upgrade without mongodb being configured.

As this charm no longer provides any endpoints to
keystone, for Queens the identity-credentials relation
is required instead of the identity-service relation.

Change-Id: I3cefc24ffc9a0e60f446fbbdff603dfad37f7baa