sshd: Don't bind-mount /sbin/init read-write

lxc-sshd was mounting itself (the template script) as /sbin/init in the
container using a writable bind-mount.

This shouldn't be needed and could lead to quite a few problems should
one of those containers overwrite /sbin/init for some reason.

Instead simply move to a read-only bind-mount which should prevent any
accidental dammage.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>