Add feature flag for Keystone policies removed in Stein

This patch set adds a new feature flag called
``removed_keystone_policies_stein`` under the configuration
group ``[policy-feature-enabled]`` for skipping Keystone
tests whose policies were removed in Stein. This feature flag
is currently applied to credentials-related policies, e.g.:
identity:[create|update|get|delete]_credential

More info on removed Keystone policies:

https://review.openstack.org/#/c/597187/16

Change-Id: Ibd16e658d0e1367b46a2d6730f2b6970a95ae221
4 files changed