Immutable user source: test_default_project_id

If the keystone user source is immutable, such as an LDAP
active directory implementation, tempest tests that try
to create or delete a user will fail. Instead of failing,
we would like them to skip. This change uses a testtools
decorator to avoid unnecessary modifications and allow those tests
to skip. In [1], I introduced the config setting that allows
this to happen.

[1] https://review.openstack.org/#/c/585536/

Change-Id: I4d928fdbf04de65a73977dbe4a818dff4cb2aa8d
Partial-Bug: 1777047
diff --git a/tempest/api/identity/admin/v3/test_default_project_id.py b/tempest/api/identity/admin/v3/test_default_project_id.py
index 302a0e5..a79cbc3 100644
--- a/tempest/api/identity/admin/v3/test_default_project_id.py
+++ b/tempest/api/identity/admin/v3/test_default_project_id.py
@@ -9,6 +9,8 @@
 #    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 #    License for the specific language governing permissions and limitations
 #    under the License.
+import testtools
+
 from tempest.api.identity import base
 from tempest import clients
 from tempest import config
@@ -32,6 +34,10 @@
         self.domains_client.update_domain(domain_id, enabled=False)
         self.domains_client.delete_domain(domain_id)
 
+    @testtools.skipIf(CONF.identity_feature_enabled.immutable_user_source,
+                      'Skipped because environment has an '
+                      'immutable user source and solely '
+                      'provides read-only access to users.')
     @decorators.idempotent_id('d6110661-6a71-49a7-a453-b5e26640ff6d')
     def test_default_project_id(self):
         # create a domain