The auth_token tests were failing randomly (depending on which tests
were run together) because the global config had options left in it
by other tests (options were added as part of auth plugin
initialization and oslo.config provides no way to remove registered
options).
This change makes it so that auth_token gets a fresh config for
every test (other than one test that requires using the global CONF
object).
Closes-Bug: 1494327
Change-Id: I647f7fade01e2a619f4a5d12a4e71897423469c1