Provide missing hover hints for instance:<type> meters

Fixes bug 1243241

Previously, the Metric dropdown list on the Stats tab of the admin/
Resource Usage panel had hover hints with a short description of each
meter, except for the instance:<type> meters which were missing.

Now, we construct the hover hints list using the current set of flavor
names as reported by nova.

Change-Id: Ieb64ee1eec43fae6036a3a97e86907b9fcc201e1
(cherry picked from commit 386d5865689aeb344182133790bdcb192789b3d2)
Merge "Provide missing hover hints for instance:<type> meters" into stable/havana