Ensure all compute meters are listed in dropdown

Fixes bug 1243298

Previously, an incomplete list of Compute (Nova) meters appeared
in the Metric dropdown on the Stats tab of the admin/Resource Usage
panel. As a result, some meters were inaccessible via the dashboard.

Now, a complete list of the current set of Compute meters is constructed
by querying all meters known to the ceilometer API and cross-referencing
with the resource IDs known to refer to instances.

Change-Id: I456bb0282ad465f0cc346e2e076545fbf1bc1346
Merge "Ensure all compute meters are listed in dropdown"