/dev/shm must be turned from a directory into a symlink to /run/shm.
The templates do this only if they find -d $rootfs/run/shm.  Since /run
will be a tmpfs, checking for it in the rootfs is silly.  It also is
currently broken as ubuntu cloud images have an empty /run.

(this should fix https://bugs.launchpad.net/ubuntu/+source/lxc/+bug/1353734)
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>