Fix test_auth isolation

When running "tox -e py27 test_auth", the first test in
test_v3_auth.TestAuthJSON would fail with an authentication error.
This is because earlier tests had already loaded up the
authentication plugins and hadn't cleared them when done. This
change makes it so that the authentication plugins are cleared in
any test loading backends so that should make sure they're always
cleared for the next test.

Change-Id: I9d47d17f1da06e65b67e092d1d733ce472d10ac5
Merge "Fix test_auth isolation"