Ceilometer -- configure vCenter support

Document how to manually configure Ceilometer to report
vCenter metering information.
This could be written up just as a work-around to the bug
but customers are requesting this information so I put the
instructions in the OpsGuide.

An entry in "Known Issues" links to the Ops Guide section.

Change-Id: I7cbe299a79b75f7acdc25b2b655930eb470bdcee
Partial-Bug: 1370700