refactor QuotaV2 import to match to other exts

fixes bug 1096486

The previous code used a special extension loading mechanism to
selectively load the Quota model is the plugin matched and object path.
This was intended to load models required by plugins, but this loading
actually occurred after the db schema was created, so the model was not
always loaded.  This fix refactors the code to make the QuotaV2 ext
behave similarly to the other extensions ensuring the models are loaded
prior to database schema creation.

Change-Id: Id7d1f7ddee69bfc4419df375366319dedc3dc439
Add migration support to Quantum

implements blueprint quantum-db-upgrades

This changeset provide database migration capabilities to Quantum by
wrapping the Alembic library.

Change-Id: I8ba3a07f5a65e0fda9c0e85ed9c07c5978c53bc7
Merge "refactor QuotaV2 import to match to other exts"
Merge "Add migration support to Quantum"