Always return a list from VolumeTableMixIn._get*

Returning None from these methods leads to TypeError
(None is not iterable)

Fixes: bug #1182075
Change-Id: Ia5f5fe78669bbaed599b7dd5cae1beb69bde7796
(cherry picked from commit decee66cb412cafecddc581cd8f229e77a9c6f1a)
Merge "Always return a list from VolumeTableMixIn._get*" into stable/grizzly
Merge branch 'stable/grizzly' of https://github.com/openstack/horizon into grizzly/router_rules