Enable tempest tests for application credentials

In Queens and later, the application credentials feature is available on
keystone and enabled by default. It should be tested in devstack.

Depends-on: https://review.openstack.org/545627
Change-Id: I4b0dc823487e79df16e1e603012ba4a7dc438389
diff --git a/lib/tempest b/lib/tempest
index 0605ffb..9de9b91 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -303,6 +303,10 @@
     # as this is supported in Queens and beyond.
     iniset $TEMPEST_CONFIG identity-feature-enabled project_tags True
 
+    # In Queens and later, application credentials are enabled by default
+    # so remove this once Tempest no longer supports Pike.
+    iniset $TEMPEST_CONFIG identity-feature-enabled application_credentials True
+
     # Image
     # We want to be able to override this variable in the gate to avoid
     # doing an external HTTP fetch for this test.