Link reference to `src/settings.sh` to the file.
Fix path to scripts dir.
Updated list of available VM images.
Set CentOS version to 'latest' for simplicity.
Update docs: `gcutil` -> `gcloud` to match code.
Remove the read-only setting on vars.

This allows us to source the settings of `$PROJECT` and `$ZONE` into other
scripts and use them as defaults with optional overrides rather than as global
constants.
Added a script and howto for using a SOCKS proxy.

Using a SOCKS proxy is much easier than individual SSH tunnels for (host, port)
pairs, especially when accessing multiple cloud VM instances, and/or when they
link to each other, as this makes it a simple, seamless user experience.
Link to the directory rather than the README.

GitHub automatically displays the README in a directory, but linking to the
directory lets the user also see the contained files and directories within.

Linking to a specific file (especially README) is confusing as there is no
navigation once the user is looking at the page, and one needs to explicitly
navigate to the directory listing to look at the files/directories further.
Switch from an SSH tunnel to a SOCKS proxy.
Use `${CLOUD_LAUNCHER}` to refer to repo root.

Also added specific instructions for running a SOCKS proxy.
Added a new modular framework for Ambari.

Starting off with supporting just CentOS 6, but this enables a reuse of
Ambari setup/install/start functionality to also support RHEL and SUSE easily.
Add note to use manual registration instead of SSH.
Build metadata cache after adding HDP repo.
Add deployment for RHEL 6 (WIP).