heat engine : subclass keystone client to encapsulate common code

heat engine : subclass keystone client to encapsulate common code

Encapsulate the keystone client in a heat-specific wrapper subclass
so we can put heat-specific implementation related to keystone in
one place.  This will allow easier reuse of common code between
resources which need to manipulate stack users and ec2 keys

blueprint metsrv-remove
Change-Id: I3d9751023c52cb75ab5e1f62415b1db4e4361dec
Signed-off-by: Steven Hardy <shardy@redhat.com>