Close #389

We will probably also want to switch the order of the mount attempts, as
the new overlay fs should quickly become the more common scenario.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: KATOH Yasufumi <karma@jazz.email.ne.jp>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>