Improve unit tests for glance.common.auth module

Add additional case to the test glance.tests.unit.test.auth.required_creds,
to verify that plugin created without required
credential pieces raises an exception

Fixes: bug #1152009
Change-Id: I3fdd9224173bdb33a9bd2ab348e22675d735099f
Merge "Improve unit tests for glance.common.auth module"