Support both centos-6-latest and centos-7-latest.

Also regenerated the VM images cache, which re-added 'centos-6-latest' image as
well as adding a new version of 'coreos-apha-*'.
Added a deployment of Gitlab on CentOS 6.
Added README for GitLab on CentOS 6.
Added GitLab to main index and local OS selection.
Move list of OS options for HDP into its own dir.
Find `settings.sh` relative to location of this script.