Add missing mock for cinder.default_quota_get

When running tests, I get the following logs:

INFO:requests.packages.urllib3.connectionpool:Starting new HTTP connection (1):
public.nova.example.com

DEBUG:cinderclient.client:Connection refused:
HTTPConnectionPool(host='public.nova.example.com', port=8776): Max retries
exceeded with url: /v1/os-quota-sets/1/defaults (Caused by <class
'socket.gaierror'>: [Errno -2] Name or service not known)

Which signify, that there is a mock missing in the tests.
This patch adds that missing mock.

Change-Id: I4f79f533b729112d11c7165adde2ac751d49f46b
Closes-Bug: #1300141
(cherry picked from commit e019ed140e31f4a25b843429b70c3e28d48a8628)