add a clone hook for ubuntu-cloud images

You can 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"