AMI-Scripts cleanup: use right URL, don't install openjdk-7-jre

Hi,

These are two very minor fixes I did some time ago, would you apply them? Do you want me to create an associated JIRA issue?

Changes:
- Documentation: the URL for downloading ubuntu-init.py points at a custom user GitHub repository (bwall). Instead make it use the script distributed by the EC2 plugin on the Jenkins server.
- The script ubuntu-ami-setup.sh requests for openjdk-7-jre install. This is useless, redundant with previous instructions and potentially wrong in some use cases. Remove that package install request.