Add netinfo.getDefaultGateway()

Used in patch:
https://bugzilla.redhat.com/1015009

On a new VDSM installation, or an old one upgrading to
3.4, we detect the current networking configuration and
persist it to files of our own format. More on this at:
http://www.ovirt.org/Feature/NetworkReloaded#Unified_persistence

It is necessary to detect the host default gateway so
the information could be persisted to files.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1015009
Change-Id: I15be5d69b5ef8f9e4cb9a30a3739a0b060225e0e
Signed-off-by: Assaf Muller <amuller@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23346
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>