network: Fix network layout discovery

Previously Node conuldn't discover all allowed network layouts. This is
nowed fixed. Additionally many unittests were added to prevent
regressions.

Change-Id: Ie9e2e226b0eb060168708756825026c36e7a3330
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=980233
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>