Updated openstack/openstack
Project: openstack/keystone  b11c072cfa75084091e9cf80f89eca7690010820

the user_tenant_membership table was replaced by "assignment"

This updates some help text to reference the new assignment table rather
than the old table.

Change-Id: I69d1ef4e788a58e3f15104ff9083cf5a4c8dc761