Fix outdated comment in KeystoneClientV3.
Fix outdated comment in KeystoneClientV3.

This patch fixes the outdated class comment for KeystoneClientV3.

Change-Id: I7735dd8b2c672cf9e40d01a6a72fd90f1540352a

dragorosson/heat,takeshineshiro/heat,gonzolino/heat,redhat-openstack/heat,steveb/heat,miguelgrinberg/heat,dims/heat,miguelgrinberg/heat,noironetworks/heat,cwolferh/heat-scratch,redhat-openstack/heat,srznew/heat,rdo-management/heat,steveb/heat,noironetworks/heat,dragorosson/heat,rdo-management/heat,pratikmallya/heat,cwolferh/heat-scratch,pshchelo/heat,cryptickp/heat,openstack/heat,cryptickp/heat,srznew/heat,openstack/heat,gonzolino/heat,takeshineshiro/heat,maestro-hybrid-cloud/heat,rh-s/heat,dims/heat,jasondunsmore/heat,maestro-hybrid-cloud/heat,rh-s/heat,pshchelo/heat,pratikmallya/heat,jasondunsmore/heat