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: Ib93207505872c7a0cc49dd33ed01239ad114cac3