always remount / rslave before running creation template (if root)

If we're not root, our mounts in private userns won't get pushed
back anyway.  If we are root, we need to make sure that anything
the template does gets cleaned up.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>