tests: miscellanous tests with/without addNetwork.

Added three functional tests:

* testTwiceAdd
* testDelWithoutAdd
* testEditWithoutAdd

covering the behaviour of addNetwork when bridge is already in use,
editNetwork and delNetwork in case of missing network.

Change-Id: I1a762983407dea3bca58e550cd5cd40758888312
Signed-off-by: Giuseppe Vallarelli <gvallare@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17401
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>