Geo: improve process to designate a secondary node as primary(2-node setups)

Right now, designating a secondary node as the primary node requires to enter a bunch of ruby command in the CLI (documentation). You have to type a bunch of commands in the rails console, and then trigger a manual rsync.

This is not easy to do. We should simplify the process of designating a primary node.

Proposal

• Provide a new CLI command gitlab-ctl promote-to-primary-node
• This command has to run from the secondary we want to promote (we should display a big warning to the user)
• We should detect if this command is run from the primary node and prevent it from running

Out of scope:

• Have a button in the Geo UI to designate a secondary node as the primary node.

Links / references

Current documentation: https://docs.gitlab.com/ee/gitlab-geo/disaster-recovery.html#promoting-a-secondary-node