Make compute manager call quota_reserve through conductor

Commit 6e8737c3aa52889394e843272f3a7e5a3a5935c1 added a quota_reserve()
call in compute manager that in turn hits the db directly.

This change adds a new conductor proxy method for quota_reserve that
the manager can use.

Change-Id: Ida84117e997bc7a8b87407efeef5072981e691a1
Closes-Bug: #1226759