Proper handling of "No default router" case

If no router is detected, the command will return NOT_FOUND instead of None.
The reason for this is that if networking.routerId is assigned to None, the workflow will attempt to detect it again and again
which causes the workflow to freeze (launchpad bug #1261356).

Now the command returns NOT_FOUND instead of None.
Workflow  now will be able to handle  this NOT_FOUND constant to properly interrupt the execution and report an error.

 This is a workaround: proper (and longer) fix will require to either fail with exception (which then should be properly handled
 in the failure block) or create a router manually. This will be addressed later in 0.4.1.
 Currently there is no way to properly handle exception sin networking commands, so scenarios which uses failure blocks are
 not applicable, so the handling should be done in success method of the command

  Partial-Bug: #1261356

Change-Id: I39acb068c89dd6e0f781147059641d0bc4f10e57