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/releasenotes/notes/removed-keystone-policies-stein-feature-flag-6cfebbf64ed525d7.yaml b/releasenotes/notes/removed-keystone-policies-stein-feature-flag-6cfebbf64ed525d7.yaml
new file mode 100644
index 0000000..3bed287
--- /dev/null
+++ b/releasenotes/notes/removed-keystone-policies-stein-feature-flag-6cfebbf64ed525d7.yaml
@@ -0,0 +1,8 @@
+---
+features:
+  - |
+    Added 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