tests the Keystone extensions V2 API

This patch adds the JSON/XML clients and tests for
Keystone discover extensions V2 API.

Change-Id: I9e869b9a5ba9302a77027581de7ae970f8d56305
diff --git a/tempest/services/identity/json/identity_client.py b/tempest/services/identity/json/identity_client.py
index 479a289..b0cab8e 100644
--- a/tempest/services/identity/json/identity_client.py
+++ b/tempest/services/identity/json/identity_client.py
@@ -27,7 +27,8 @@
         self.endpoint_url = 'adminURL'
 
         # Needed for xml service client
-        self.list_tags = ["roles", "tenants", "users", "services"]
+        self.list_tags = ["roles", "tenants", "users", "services",
+                          "extensions"]
 
     def has_admin_extensions(self):
         """
@@ -237,6 +238,12 @@
         resp, body = self.put('users/%s/OS-KSADM/password' % user_id, put_body)
         return resp, self._parse_resp(body)
 
+    def list_extensions(self):
+        """List all the extensions."""
+        resp, body = self.get('/extensions')
+        body = json.loads(body)
+        return resp, body['extensions']['values']
+
 
 class TokenClientJSON(IdentityClientJSON):