Support quota in mogan(part two)

This patch introduces the quota operation in
creating and deleting instances process.

Change-Id: If6573fafc5acb805cf727acdc8f0f9872fc1a717
Implements: blueprint quota-support

mogan/conf/quota.py

@@ -20,6 +20,7 @@ from mogan.common.i18n import _
 
 quota_opts = [
     cfg.StrOpt('quota_driver',
+               default="database",
                help=_("Specify the quota driver which is used in Mogan "
                       "service.")),
     cfg.IntOpt('reservation_expire',