Fix condition mistake for limit argument check

in file glance/api/v2/metadef_namespaces.py, Line 468

468 if limit < 0:
469 msg = _("limit param must be positive")
470 raise webob.exc.HTTPBadRequest(explanation=msg)

I think ignore the zero condition and according to #http://developer.openstack.org/api-ref-image-v2.html
explain (Requests a specified page size of returned items from the query. Returns a number of items up to the specified limit value. Use the limit parameter to make an initial limited request and use the ID of the last-seen item from the response as the marker parameter value in a subsequent limited request. )

I think Line468 it should change to 'if limit <= 0:'