Make cinderclient a more optional dependency
Make cinderclient a more optional dependency

This catches import errors around cinderclient imports in the cinder store.
Since python-cinderclient was moved to test-requirements.txt, it is not
installed in the main glance gate jobs. This caused a lot of errors in the
logs, which should be avoided.

Change-Id: Iec6ac7020f66f9f9728d6cf4452127b8d55a8b2d
Closes-Bug: #1471063

openstack/glance_store,redhat-openstack/glance_store,openstack/glance_store,cpallares/glance_store,cpallares/glance_store,redhat-openstack/glance_store