feat(contrib): auto-create discovery url in ec2 provision

* The provision-ec2-cluster.sh now creates discovery_url on
  the fly instead of the user needing to create it manually
* The discovery_url is stored in a file /tmp/discoveryurl.txt
* Extra Comments to break down the file into sections.
* Readme section for asking to update discovery_url no longer
  present.
ah just what i was looking for! thanks. I will make that change and resubmit.