webadmin: Added gateway format validation

When editing the management network in the host network setup dialog,
added a check that the default gateway field is formatted as a valid IP
address.

Change-Id: I272285019b5e87d8324c2192ea601aeed91cdbbf
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=890177
Signed-off-by: Lior Vernia <lvernia@redhat.com>