Fix manila devstack plugin for keystone v3 usage
Fix manila devstack plugin for keystone v3 usage

A devstack run with a keystone v3 only setup (ENABLE_IDENTITY_V2=False)
fails currently.

Partial-bug: #1553082
Change-Id: Ie8885d2c2f4d82d9894f4ca233153184125c20f2

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