Skip nova cli tests with volumes if Cinder unavailable

The nova cli tests for volumes will fail if Cinder is
not being tested. As it is allowable to skip Cinder tests,
we should skip these tests in that eventuality.

Change-Id: Ia3c82d416962dad446390b82780978ddaf03f1b2
Closes-Bug: #1288373
Merge "Skip nova cli tests with volumes if Cinder unavailable"