Switched from `gcutil` to `gcloud`.

`gcutil` has been deprecated in favor of `gcloud compute`:
https://developers.google.com/compute/docs/gcutil/
Target instance -> `localhost` for SSH endpoint.

For SSH forwarding, the `ssh` command syntax is

    `ssh -L [bind_address:]port:host:hostport [...]`

where the `host` is relative to the host we have connected to, which is
`${SERVER}` in this case. Thus, the simplest endpoint is `localhost`
since we don't need to make another hop.
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.
Fix capitalization: `s/Gitlab/GitLab/`.
Update paths due to recent file moves and new dirs.
Fixed capitalization: `s/Gitlab/GitLab/`.
No need to install SSH; GCE VMs already have it.
Update to refer to project root dir symbolically.
Added GitLab deployment on CentOS 7.
Added link to the CentOS 7 deployment.
Added a WIP of GitLab on Debian 7 deployment.
Added `bunch` to provide `dict`-like Python class.

This will replace a custom-written class to handle the same thing, which allows
both `a.b` and `a['b']` for references to fields in JSON-like objects.
Replaced custom `Dict` class with `Bunch`.

Also disambiguated the difference between `$PYTHONPATH` (search path for Python
files, which is a list of paths like `$PATH`) from the source path (where our
Cloud Launcher core source code is found) which is now `$SOURCE_PATH`, and is
now just one of the directories in `$PYTHONPATH`.
Add a blank line before first top-level class.
Remove commented out setting for `$PYTHONPATH`.