This patch lets the quota engine verify whether it manages a resource
for which a limit is being checked.
So far this check has been delegated to the drivers. This is
conceptually wrong and also lead to code duplication.
Unit tests and some docstrings are also fixed accordingly as a
part of this patch.
Closes-Bug: #1430519
Related to blueprint better-quotas
Change-Id: If1467881f13e30afa53a23b904f8eae5c7264834