Management network is now kept in main routing table

Only the management network should add its default route
to the main routing table. Each NetDevice holds an IpConfig
instance who now holds defaultRoute='yes', 'no' or None.
Only the top level device will have a defaultRoute value.
When writing ifcfg files we check if we got a defaultRoute
value, if so we write it.

Patch Set 7 changed the implementation so that only the top
level device has DEFROUTE=yes/no in its ifcfg file.

Change-Id: I55c50269dd3d52fd058951200282c925a7014aca
Signed-off-by: Assaf Muller <amuller@redhat.com>
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=984028
Reviewed-on: http://gerrit.ovirt.org/17575
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>