busybox template: support for unprivileged containers

Apply the changes found in templates/lxc-download to the busybox template as
well. Change ownership of the config and fstab files to the unprivileged user,
and the ownership of the rootfs to root in the new user namespace.

Eliminate the "unsupported for userns" flag.

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@freescale.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
busybox template: mount fstab when available

When running unprivileged, lxc-create will touch a fstab file, with bind-mounts
for the ttys and other devices. Add this entry in the container config.

Signed-off-by: Bogdan Purcareata <bogdan.purcareata@freescale.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>