Added install instructions for RackSpace cloud using Ubuntu 12.04, 12.10, 13.04
updated the rackspace docs to have more information, and fixed a couple of mistakes
fixed the git clone url, it was the read-write one that required ssh access to the docker account, changed to the read only version
Merge pull request #588 from kencochrane/rackspace-docs

+ Documentation: Added install instructions for RackSpace cloud using Ubuntu 12.04, 12.10, 13.04 and fixed a few minor issues with the ubuntu install docs.