Improve testing of list_extensions for compute

This commit takes the testing for list_extension from the volumes
extensions client and uses it for the compute testing of
list_extensions. This also fixes a skip decorator to use use
requires_ext instead of calling is_extension_enabled directly.

Change-Id: Id08f39e09df0aea0d3c8f507f421c82ddf32900f
diff --git a/tempest/services/compute/xml/extensions_client.py b/tempest/services/compute/xml/extensions_client.py
index b17fc4f..8b7b90a 100644
--- a/tempest/services/compute/xml/extensions_client.py
+++ b/tempest/services/compute/xml/extensions_client.py
@@ -37,7 +37,7 @@
         url = 'extensions'
         resp, body = self.get(url, self.headers)
         body = self._parse_array(etree.fromstring(body))
-        return resp, {'extensions': body}
+        return resp, body
 
     def is_enabled(self, extension):
         _, extensions = self.list_extensions()