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.
What's the issue between gnu-sed and darwin's sed here? I don't think the EC2 install script should be installing a different version of `sed`, and especially not with homebrew (not everyone has homebrew installed). If it's a difference between the `-i` option and the `-l` option, [there are ways to get around that](https://github.com/heroku/heroku-buildpack-python/blob/5cda51c7a58d42c15c406e4afce91d503a39d636/bin/utils#L3-L7).