clone hook support for ubuntu cloud template

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.