OVSDB L3 support: Allow for override of default gateway mac address

With this change we are introducing a new system config that will be used by ovsdb's l3
implementation: ovsdb.l3gateway.mac

Its good to note here that until bug 1705 is fixed, this config will be needed so that
initial implementation is able to route packets to subnets outside the neutron l3
subnets.

Change-Id: Iaca1be3651aeff92b789b451df12e77ba842508e
Signed-off-by: Flavio Fernandes <ffernand@redhat.com>
Merge "OVSDB L3 support: Allow for override of default gateway mac address"