Add upgrade script

upgrade.sh upgrades to a new snapshot like so:
* applies rjil::jiocloud::sources to construct a sources.list
  corresponding to the snapshot version.
* apt-get update
* apt-get install jiocloud-puppet (thus upgrading the puppet code)
* applies rjil::jiocloud::preupgrade in case something needs to be
  tweaked before apt-get dist-upgrade
* apt-get dist-upgrade
* Run puppet apply
* Record the new snapshot version in a file in /etc