Geo: Add script to add a secondary site

What does this Merge Request do and why?

Make it easy to add a secondary site. This is a stepping stone to make it easy to install Geo from scratch.

If you have one working GDK already: How to add a secondary site

Replace /home/mike/ with the actual path to the parent directory of your existing GDK directory.

cd /home/mike
git clone https://gitlab.com/gitlab-org/gitlab-development-kit.git gdk2
cd /home/mike/gdk
./support/geo-add-secondary --secondary-port 3001 --primary . ../gdk2

You can also test this MR by running !2475 (merged). It has run successfully for 2 Geo developers other than myself.

Merge Request checklist

• Modify doc/howto/geo.md
• This change is backward compatible. If not, please include steps to communicate to our users.
• [-] Tests added for new functionality. If not, please raise Issue to follow-up. => #1233 (closed)
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.