module.lxc: support for complex network scenariis.

This allow to set additional network nics where the gateway can be setup on.
This allow for exemple eth0 to be on an hostonly network while eth1
is wired to the physical host throught another bridge and eth1 is holding
a publicly addressable IP.

cc @mgw @thatch45 @s0undt3ch @regilero @squizou @jpcw @samaradona
lxc: override default gw
Merge pull request #14552 from makinacorpus/lxc

[only review atm, do not merge] module.lxc: support for complex network scenariis.