Skip tests in UsersV3TestJSON with an immutable user source 

Adds a skip_checks condition to UsersV3TestJSON to skip if immutable 
user source is true. resource_setup() in this class attempts to create
users, which will fail if the environment uses an LDAP directory for 
example.

Change-Id: I42c3b7a17bfb73079b1d05951636359324bed207
diff --git a/tempest/api/identity/admin/v3/test_list_users.py b/tempest/api/identity/admin/v3/test_list_users.py
index c69e4c8..5aec931 100644
--- a/tempest/api/identity/admin/v3/test_list_users.py
+++ b/tempest/api/identity/admin/v3/test_list_users.py
@@ -34,6 +34,14 @@
                          map(lambda x: x[key], body))
 
     @classmethod
+    def skip_checks(cls):
+        super(UsersV3TestJSON, cls).skip_checks()
+        if CONF.identity_feature_enabled.immutable_user_source:
+            raise cls.skipException('Skipped because environment has an '
+                                    'immutable user source and solely '
+                                    'provides read-only access to users.')
+
+    @classmethod
     def resource_setup(cls):
         super(UsersV3TestJSON, cls).resource_setup()
         alt_user = data_utils.rand_name('test_user')