Fix sluggish rbd unit tests
Fix sluggish rbd unit tests

The rbd_test.py module tests methods decorated by utils.retry,
which does retries with exponential backoffs, without mocking out
time.sleep().

This commit mocks time.sleep(), yielding a 3x speedup, as documented
in the accompanying bug.

Closes-Bug: #1536717

Change-Id: I414629f09f397e44d3a5be794ef83658a4c1eec0

j-griffith/cinder,mahak/cinder,dims/cinder,openstack/cinder,j-griffith/cinder,bswartz/cinder,Datera/cinder,openstack/cinder,bswartz/cinder,cloudbase/cinder,NetApp/cinder,phenoxim/cinder,eharney/cinder,Hybrid-Cloud/cinder,eharney/cinder,Nexenta/cinder,mahak/cinder,Datera/cinder,phenoxim/cinder,ge0rgi/cinder,dims/cinder,cloudbase/cinder,Hybrid-Cloud/cinder,NetApp/cinder,Nexenta/cinder