iproute2: Add source routing support

Up until now, only the ifcfg configurator had support for source
routing, i.e., adding a routing table for networks that want to
set a non default (system-wide) gateway.

This patch implements the needed pieces for iproute2 keeping with
the current infrastructure of vdsm/network/sourceroute.py, although
I foresee future patches for simplifying the pieces of the puzzle.

Change-Id: Ic182b55ced564f69e71313d149042a9685b4178d
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27239
Reviewed-by: Dan Kenigsberg <danken@redhat.com>