device/device/utils/network: Add support for MTU inheriting from parent on bridged devices
device/device/utils/network: Add support for MTU inheriting from parent on bridged devices

If custom mtu setting is not supplied, and parent is supplied, then MTU is inherited from parent device.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

hallyn/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,Skarlso/lxd,mjeanson/lxd,mjeanson/lxd,stgraber/lxd,stgraber/lxd,atxwebs/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,atxwebs/lxd,atxwebs/lxd,stgraber/lxd,mjeanson/lxd,Skarlso/lxd,lxc/lxd,stgraber/lxd,Skarlso/lxd,atxwebs/lxd,Skarlso/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd