lxc3 deprecates many legacy config keys [1].
This change ensures that containers created on systems with lxc3
use the appropriate apparmor config keys. At this point we do not
need to address lxc2->lxc3 upgrades as these would only occur
during a Xenial->Bionic in-place OS upgrade, which is an unsupported
upgrade path.
[1] https://discuss.linuxcontainers.org/t/lxc-2-1-has-been-released/487
Change-Id: I9f30339210827f90818ea6993d90ca68c17fd3b2
440c87a808