Make image metadef tests admin-only

Test the image metadef API with admin credentials, which is really
how they should be deployed anyway. Glance is moving to make this the
default policy, so tempest needs to do the same.

Related-Prod: PRODX-28766
Related-Bug: #1916926
Change-Id: Ie284e87236191f91f388fb1530db9881cca338b8
6 files changed