Geo : Add a single command to promote a secondary node

What does this MR do?

Adds a single command to promote a Geo node in any of the following architectures:

• Failing over to a secondary node with a single node;
• Failing over to a secondary node with multiple nodes;
• Failing over to a secondary node with a Patroni standby cluster;

Proposal

Usage:
  gitlab-ctl geo [options] command [options]

GLOBAL OPTIONS:
  -h, --help      Usage help
  -q, --quiet     Silent or quiet mode
  -v, --verbose   Verbose or debug mode

COMMANDS:
  ..
  promote               Promote the current Geo node
  ...

Related issues

gitlab#335920 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened