Test with EE load balancing code enabled

https://gitlab.com/gitlab-org/gitlab-ee/issues/8692 was not caught until 11.6.x was deploying to staging, and there were no integration tests for it.

I actually have this enabled in my Geo dev environment by just pointing my database.yml to the secondary DB:

development:
  adapter: postgresql
  prepared_statements: false
  encoding: unicode
  database: gitlabhq_development
  pool: 6
  host: /home/stanhu/gdk-ee/postgresql
  load_balancing:
    hosts:
      - stanhu-geo-secondary

We could do some basic verification that GitLab still works by doing:

1. Set up a secondary PostgreSQL database using the Geo QA setup
2. Change database.yml to have the primary node use the secondary DB