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
diff --git a/devstack/plugin.sh b/devstack/plugin.sh
index a6259f4..502b68c 100644
--- a/devstack/plugin.sh
+++ b/devstack/plugin.sh
@@ -27,10 +27,10 @@
         iniset $TEMPEST_CONFIG policy-feature-enabled volume_extension_volume_actions_reserve_policy False
         iniset $TEMPEST_CONFIG policy-feature-enabled volume_extension_volume_actions_unreserve_policy False
 
+        # TODO(cl566n): Remove these once stable/pike becomes EOL.
         # These policies were removed in Stein but are available in Pike.
         iniset $TEMPEST_CONFIG policy-feature-enabled removed_nova_policies_stein False
-
-        # TODO(cl566n): Policies used by Patrole testing. Remove these once stable/pike becomes EOL.
+        iniset $TEMPEST_CONFIG policy-feature-enabled removed_keystone_policies_stein False
         iniset $TEMPEST_CONFIG policy-feature-enabled added_cinder_policies_stein False
     fi
 
@@ -39,16 +39,17 @@
             RBAC_TEST_ROLE="Member"
         fi
 
+        # TODO(cl566n): Remove these once stable/queens becomes EOL.
         # These policies were removed in Stein but are available in Queens.
         iniset $TEMPEST_CONFIG policy-feature-enabled removed_nova_policies_stein False
-
-        # TODO(cl566n): Policies used by Patrole testing. Remove these once stable/queens becomes EOL.
+        iniset $TEMPEST_CONFIG policy-feature-enabled removed_keystone_policies_stein False
         iniset $TEMPEST_CONFIG policy-feature-enabled added_cinder_policies_stein False
     fi
 
     if [[ ${DEVSTACK_SERIES} == 'rocky' ]]; then
         # TODO(cl566n): Policies used by Patrole testing. Remove these once stable/rocky becomes EOL.
         iniset $TEMPEST_CONFIG policy-feature-enabled added_cinder_policies_stein False
+        iniset $TEMPEST_CONFIG policy-feature-enabled removed_keystone_policies_stein False
     fi
 
     iniset $TEMPEST_CONFIG patrole rbac_test_role $RBAC_TEST_ROLE