Fix API inconsistence when changing node's states

When the request to change the power state is accepted by the server
(HTTP 202) it shouldn't return any response message, just like the
request to change the provision state. In the future after WSME add
the support to edit the Header Location, we should include a link to
the /nodes/<uuid>/states subresource so that clients know how to
track the status of the request, there's a FIXME in the code
mentioning that already.

The patch also add a check to the provision_state tests to make sure
the response body has no content.

Change-Id: I4fb1cd5c226cee2f9e0977e83cf2ffa7786b3b4a
Closes-Bug: #1270160
Merge "Fix API inconsistence when changing node's states"