Migrate Geo Tracking database configuration into database.yml

Overview

Configuring a Geo Tracking database is done using the geo_secondary[] configuration options.

Behind the scenes, this populates geo_database.yml.

It needs to instead configure database.yml using the multi-database configuration methodology recently enabled by !5480 (merged).

Out of scope

Moving to unified configuration methods and options is part of #6306 (closed) and a separate task.

Definition of done:

1. when geo_secondary[] fields are configured in gitlab.rb, they should populate database.yml instead of geo_database.yml
2. new Geo deployment works as expected with Geo Tracking in database.yml
3. upgrading from existing deployment using geo_database.yml to package that uses database.yml works smoothly