Update python-ceilometerclient to support Keystone V3 API

Keystone is now deprecating the V2 API in favor of the V3 API.
The ceilometer client is now using the keystone session to
delegate the discovery of the version that is going to be used,
this eliminates the need of updating the client every time there
is a new API version.

DocImpact

blueprint support-keystone-v3-api

Change-Id: I6ebacce7adf70f24bdede9b920853ab9851071cf
Merge "Update python-ceilometerclient to support Keystone V3 API"