add a clone hook for ubuntu-cloud images

This allows ability to now specify '--userdata' arguments to 'create' or
to 'clone'. So now, the following means very fast start of instances with
different user-data.

$ sudo lxc-create -t ubuntu-cloud -n precise -- \
   -r precise --arch amd64

$ sudo lxc-clone -B overlayfs -o precise -s -n ephem1 \
   --userdata="my.userdata1"
$ sudo lxc-clone -B overlayfs -o precise -s -n ephem2 \
   --userdata="my.userdata2"

Also present here is
 * an improvement to the static list of Ubuntu releases. It uses
   ubuntu-distro-info if available degrades back to a static list on failure.
 * moving of the replacement variables to the top of the create template This
   is just to make it more obvious what is being replaced and put them in a
   single location.

Signed-off-by: Scott Moser <smoser@ubuntu.com>