lxc-template: enable chroot + chpasswd functionality for Busybox hosts

This patch supports the scenario where a user wants to install a
busybox container on a busybox host.

When running the template, in order to change the root password,
the template needs to do the chroot. On busybox-powered hosts, chroot
is not part of the coreutils package - it's part of busybox. And the
busybox implementation or chroot only works if it has /lib in the new
root populated with the right binaries (or at least that's the
solution I found to make it work).

The temporarily bind-mounts /lib in the NEWROOT, chroots there,
changes the password, goes back and unmounts. This set of operations
is contained in a new MOUNT namespace, using the lxc-unshare call.

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