Refactor common auth token code in images test
Refactor common auth token code in images test

This pulls out the authentication token manipulation used by some of the
images functional tests, which is replicated in several places into a
single helper. This will also be used by a subsequent functional test for
cross-owner copy-image support.

Change-Id: I80cf190f122ce61a47340181e1ad02ee24b5cb2d

openstack/glance,openstack/glance,openstack/glance