lxc-start-ephemeral: Set tmpfs mode to 0755

The tmpfs was mounted with its default mode (1777) which was then picked
up by overlayfs/aufs as the target's mode.

This led to a world writable / in ephemeral containers.

I have confirmed that this issue doesn't impact lxc-clone.

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