Fix legacy alarm matching_metadata broken by recent fix.

The prefix was being computed incorrectly because meter_name is now
being stripped from kwargs. This patch computes the prefix before
kwargs is modified.

Change-Id: I858dd430a4ce4be5cda41b3dd7faa76b8fdf2cfe
Closes-Bug: #1383521
Imported Translations from Transifex

For more information about this automatic import see:
https://wiki.openstack.org/wiki/Translations/Infrastructure

Change-Id: If10ddacdbc30777b2f24fd7e40bf5210da39f5ef
Merge "Fix legacy alarm matching_metadata broken by recent fix."
Merge "Imported Translations from Transifex"