Documenting hypervisor support for nova meters

Support was recently added for HyperV and Vsphere for Nova meters in
ceilometer. However, not all meters are supported by all the
hypervisors. Hence, have added documentation to list the meters and the
hypervisors which provide support for them.

Change-Id: I9c828ffdd8276a487a35f31ba4ba11dcab93964c
Implements: blueprint vmware-vcenter-server
Merge "Documenting hypervisor support for nova meters"