Geo: Let sysadmins run FDW queries from the tracking DB

Same problem as https://gitlab.com/gitlab-org/gitlab-ee/issues/10348.

In an HA setup, with the tracking DB on its own machine, you can't connect to the tracking DB and run FDW queries without jumping through a bunch of hoops.

Typical frustrating experience:

• SSH into tracking DB machine
• sudo gitlab-geo-psql
• Run some query that includes references to gitlab_secondary (which is the foreign server)
• ERROR: user mapping not found for "gitlab-psql"
• Oh, I need to be gitlab_geo user since it created the foreign server
• \q
• sudo gitlab-geo-psql -U gitlab_geo
• psql: FATAL: Peer authentication failed for user "gitlab_geo"
• Oh, we configured PG to not allow peer authentication, I'll connect from an app node
• SSH into app node
• sudo gitlab-geo-psql
• psql: could not connect to server: No such file or directory Is the server running locally and accepting connections on Unix domain socket "/var/opt/gitlab/geo-postgresql/.s.PGSQL.5431"?
• Oh, the command doesn't do what you'd hope
• gitlab-geo-psql -U gitlab_geo -h 10.138.0.38
• Password for user gitlab_geo:
• Oh I need to get the password (if you're very familiar with GitLab you know it's in plaintext somewhere in /etc/gitlab/gitlab.rb but you get the point)

Let's make this easier.

cc @brodock