Provide a command to check that secondary can access primary
From Sid's demo:
Step 8 in https://docs.gitlab.com/ee/gitlab-geo/database.html#step-1-configure-the-primary-server says
Check to make sure your firewall rules are set so that the secondary nodes can access port `5432` on the primary node.
We should provide a command that can be run to check this. And we should check if similar steps exist further along that can also use such a command.