Add valid check and unit tests on quota class

1.Cinder quota class hard_limit’s value should larger than -1 (-1 is a flag
value for unlimited).
2.Add unit tests for quota class.
3.Add quota_classes extension enabled in unitest, so as to do
unitest for quota_classes' update operation.

Closes-bug: #1245360

Change-Id: Ie483db1bdbd4fac875de26abab8ebebd96f4938b
(cherry picked from commit 8b6d51ea04a65d846b0e92a11e191327360d7427)
Merge "Add valid check and unit tests on quota class" into stable/havana