Support devstack install without nova
Support devstack install without nova

Many Manila drivers don't require Nova, but the devstack
plugin still depends on Nova. This change allows using
the devstack plugin without Nova.

Change-Id: Ie76300f5af605365e327e7528a6d5aefa85a74fe

NetApp/manila,NetApp/manila,bswartz/manila,bswartz/manila,openstack/manila,vponomaryov/manila,openstack/manila,vponomaryov/manila