Use ActiveRecord::Tasks::DatabaseTasks directly in geo rake tasks

Instead of piggybacking on the existing `db:` rake tasks, redefine
them and correctly set all the variables.

Closes gitlab-org/gitlab-ee#3543

changelogs/unreleased-ee/tc-geo-use-database-tasks.yml

+---
+title: Rewrite Geo database rake tasks so they operate on the correct database
+merge_request: 3019
+author:
+type: fixed