Assume compute service_type for nova client

This change removes the ability to specify a non-compute service
type when requesting a nova client. The only non-compute service
type actually requested was the fallback path for cinder. However
nova volumes was completely removed from nova in grizzly, so we
do not need to support this any more.

Change-Id: I9e15f31979d44f018db69ebc8109f707b0f51671
Merge "Assume compute service_type for nova client"