Geo: Use "geo_node_name" when defining the node as a primary during the inital setup

When setting up a PostgreSQL replication on the Geo primary node we need to define the node as a primary node running the following command:

gitlab-ctl set-geo-primary-node

This command will use the external_url defined in /etc/gitlab/gitlab.rb file to create a new GeoNode instance. We should update this command to use the Geo geo_node_name as well.

/cc @geo-team