Make quotas APIv3 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: I74295a6f8ebca46421eafedbb28e677d2361c2b8
Merge "Make quotas APIv3 extension use Quotas object for create/update"