Enrich voltracker's volume api

* Add the volume operation WSGI controller. The Restful reperentation is
  as follow:
  GET /volumes -- Returns a set of brief metadata about volumes
  HEAD /volumes/<ID> -- Returns detailed metadata about volumes with id
                        <ID>
  GET /volumes/query -- Query and search volumes metadata about volumes
                        matching specific conditions. Because the client
						uses this result to build the iscsi connections,
						in order to promote overall r/w performance and
						limit the number of connections, executor always
						returns partial matching list.
  POST /volumes/<ID> -- Register a new volume and store metadata with id
                        <ID>
  DELETE /volumes/<ID> -- Delete all tracked volume with id <ID>
  DELETE /volumes -- Delete	all tracked volumes
* Remove the previous designed 'PUT /volumes/<ID>' interface.