Allow cells to use main GDK config with cell specific tweaks

What does this merge request do and why?

This MR enables the cells integration in GDK to use main GDK config and also override cell specific configuration options.

Closes #2156 (closed).

How to set up and validate locally

1. gdk config set cells.instance_count 1
2. gdk cells up
3. Review the diff: diff -u gdk.yml gitlab-cells/cell-1/gdk.yml

Example

...
cells:
  enabled: true
  instance_count: 1
  instances:
    - name: cell-1
      config:
        gitlab:
          rails:
            session_store:
              session_cookie_token_prefix: abcd

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.