apparmor: Allow slave bind mounts

Without this, if the system uses shared subtrees by default (like systemd), you
get a large stream of

  lxc-start: Permission denied - Failed to make /<mountpoint> rslave
  lxc-start: Continuing...

with

  apparmor="DENIED" operation="mount" info="failed flags match" error=-13
  profile="/usr/bin/lxc-start" name="/" pid=17284 comm="lxc-start" flags="rw, slave"

and eventual failure plus a lot of leftover mounts in the host.

https://launchpad.net/bugs/1325468