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
(cherry picked from commit 264a595cba4e6a7a01cecb64024b18d67971eac4)
Merge "Ensure all compute meters are listed in dropdown" into stable/havana