Fix ReservationTestCase on PostgreSQL

ReservationTestCase setUp() uses a hard coded id value
of quota usage, which might be different if a DB is not
recreated for each test case. Refactor tests a bit to use
a common setUp().

Blueprint: db-api-tests-on-all-backends

Co-Authored-By: Roman Podoliaka <rpodolyaka@mirantis.com>
Change-Id: I7e8ce94fa36392a508375ca3626ff0a8441a7066
Merge "Fix ReservationTestCase on PostgreSQL"