if ubuntu-10.04.4-server-amd64.iso exists; use it for creating stemcell

if `/var/vcap/store/stemcells/ubuntu-10.04.4-server-amd64.iso` exists then assign it to `$UBUNTU_ISO` to speed up stemcell creation

OR

If creating custom stemcell, assume the user will create multiple, and will benefit from having the ISO downloaded; so just download it. `  wget http://releases.ubuntu.com/lucid/${ISO_NAME}`