Fix 'null' response on router-interface-remove operation

Fix 'null' response on router-interface-remove operation

To avoid a 'null' response body, the delete operation returns a response
that is consistent with its add counterpart, i.e. we return the router
id with details about the interface being affected by the operation, as
well as the tenant id.

A unit test is added to ensure that the right body is returned and minor
adjustments have been made to the plugins affected by the change.

Long-term, a delete operation should really return 204 w/o a body, but
this requires some major rework of the WSGI handling within Quantum.
This is an interim solution that deals with an 'ugly' response body,
whilst keeping backward compatibility.

Fixes bug 1173284

Change-Id: Icaab87ad0c8561c0690c8f0a14db815d8886bc71