Make quotas APIv2 extension use Quotas object for create/update

This makes the quotas extension use the Quotas object for the
create/update operations instead of direct database access.

Related to blueprint compute-manager-objects-juno

Change-Id: Ia2db239d721c9ea680e11409e5eefc59e0dee941
Merge "Make quotas APIv2 extension use Quotas object for create/update"