Correct docstring in load_plugin_from_args

apiclient/auth.py load_plugin_from_args docstring :raises: line does not
match the method's actual behavior. The method appears correct, so this
patch corrects the docstring.

Change-Id: I0acefd91682bab778800b92d72bc062806773355
Closes-bug: #1274660
Fix policy tests for parallel testing

After enabling parallel testing for most of oslo-incubator, I
found that a number of the policy tests were failing consistently
due to order-dependent behavior in them.  This change fixes all
of the issues that I saw in local testing.

Change-Id: I670415d616db433d17186f496f43c961aaeb2e9b
Closes-Bug: #1275150
Related-Bug: #1272035
Merge "Fix policy tests for parallel testing"
Merge "Correct docstring in load_plugin_from_args"