Add support script to add 2nd GDK directory as secondary cell

What does this merge request do and why?

Adds a script to automate creation of a 2nd GDK to act as a Cell. This script is inspired from support/geo-add-secondary.

1. Experimental!: Treat the 2nd GDK you create as throw-away.
2. There is some likelihood the 2nd GDK can accidentally corrupt data in your 1st GDK.
3. In the 2nd GDK, we skip seeding. Follow-up issue: gitlab#412075 (closed)
4. We also skip validating database config. Follow-up issue: gitlab#412280
5. I have not created a shared Redis cluster yet
6. Depending on your config, various services might clash with ports. We know sshd clashes so we disable sshd in the 2nd GDK

Related issue: gitlab#388278 (closed)

How to set up and validate locally

1. Run support/cells-add-secondary --help, and follow the help message

It should create a 2nd GDK capable of reading and writing to the primary GDK's application_settings table.

See !3142 (comment 1401848480) for an example where this worked

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.