Improve no_snapshot_gb_quota description
Improve no_snapshot_gb_quota description

Small improvemente to the description of the ``no_snapshot_gb_quota``
configuration option to clarify what it does and its default.

TrivialPatch

Change-Id: Iad01d3b011472716a5429219f6c75be2ff670cab

mahak/cinder,openstack/cinder,openstack/cinder,mahak/cinder