Fixes wrong hrefs when getting set of messages

Based on current implement, an additional path /messages will be
added, see http://git.openstack.org/cgit/openstack/marconi/queues
/transport/wsgi/messages.py#L208
However, the /messages path has been already included in the URL.
So this fix just remove it and use req.path directly.

Fixes bug 1240897

Change-Id: Ie4bf6d3df62a5b861a8fe8c6392eb8bd6bf57c2c