Implement ceilometer client plugin

This moves the client creation code out of Clients._ceilometer() into
its own client plugin.

Ceilometer is an integrated project, and python-ceilometerclient
is a dependency, so the import is now mandatory.

Change-Id: Id4961c4d9678d7bfe3739128843c41921cc8a34b
Merge "Implement ceilometer client plugin"