For veth and macvlan networks, this can look up the host address on the
bridge (link) interface and add a default route on the guest to that
address. This facilitates a typical setup where guests are bridged
together.

syntax:
	lxc.ipv4.gateway = auto
	lxc.ipv6.gateway = auto
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>