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.