Removed unused constant in v3/views/clusters.py
Removed unused constant in v3/views/clusters.py

The constant _collection_name is unused in api/v3/views/clusters.py,
removing it.

TrivialFix

Change-Id: I80154afb9e7dae2cd8134c9d415b5d26435c8bbd

eharney/cinder,Datera/cinder,phenoxim/cinder,mahak/cinder,mahak/cinder,openstack/cinder,openstack/cinder,j-griffith/cinder,phenoxim/cinder,eharney/cinder,j-griffith/cinder,Datera/cinder