Don't override images client in compute admin tests

Tests should be explicit about their use of admin credentials. The
base compute admin tests class overrode self.image_client with the
admin version. Stop doing that, and introduce a new
self.admin_image_client client that tests can explicitly use.

Needed-by: https://review.opendev.org/c/openstack/whitebox-tempest-plugin/+/805300
Change-Id: I1a0c84b41b1b77d3a618a8f15448909bd12a0742
2 files changed