Rework the apparmor lxc profile abstraction to mimic ubuntu's container-default.
This profile allows quite a lot, but strives to restrict access to
dangerous resources.

Removing the explicit authorizations to bash, systemd and cron files,
forces them to keep the lxc profile for all applications inside the
container. PUx permissions where leading to running systemd (and others
tasks) unconfined.

Put the generic files, network and capabilities restrictions directly
in the TEMPLATE.lxc: this way, users can restrict them on a per
container basis.

use_apparmor() was first designed to be called from withing libvirtd,
but libvirt_lxc also uses it. in libvirt_lxc, there is no need to check
whether to use apparmor or not: just use it if possible.