skip image tag tests
skip image tag tests

the command ``image set tag <tag> <image>`` is now failing
skip the test so our gate isn't broken.

Change-Id: I05e415b212e76aef62c1b0b966863573eff338e9
Partial-Bug: #1596573

openstack/python-openstackclient,dtroyer/python-openstackclient,dtroyer/python-openstackclient,openstack/python-openstackclient