Fix downgrade in 002_quota_class.py for MySQL

Downgrade in the script 002_quota_class.py is failing for MySQL
because there is an attempt to delete the quota_classes table
while the reservations table still has a foreign key defined.
The foreign key causes the delete of the quota_classes table to fail.

This change is based upon commit 8328fc46d783f4ec9286eededafa91afae89cba0
which makes a similar change to 018_add_qos_specs.py .

Change-Id: I81844a2da4fb3b831f1f9dead3634e82f54e559e
Closes-bug: 1265944
Merge "Fix downgrade in 002_quota_class.py for MySQL"