The lxc_host role overrides the configuration
done by the system administrator on the host.
If mtu was defined in /etc/lxc/ and in the
system network interfaces, then these configurations
will be overriden.
This commit should allow the deployer to set the
mtu for its lxcbr0 network, by setting the variable
lxc_net_mtu.
Fixes-Bug: #1518303
Change-Id: I42ab3d0a2c20ae94335de195cdb14579d9f59f5f
513244b56b