Correct api return code for invalid urls

This patch changes the server response from 501 to 404 when a request
is made to a url that is unknown. This behavior makes the behavior in
congress also consistent with how the other openstack apis function.

Change-Id: I537aa3a9b8229fbeea5fba4dcb1a8c7e905bf7d3
Closes-bug: 1366177
Merge "Correct api return code for invalid urls"