Describes Ceilometer support for vCenter

Since 6.0 release supports Ceilometer for vCenter,
the limitation on using Ceilometer must be replaced with
the feature description and implementation details.

Change-Id: Id4399c54fa85b0ae3c703e2aba444223a6cc5708
Implements: blueprint ceilometer-support-for-vcenter
Merge "Describes Ceilometer support for vCenter"