Improve GitLab Geo replicated database support

This is not DR support, its regular Geo. Since last time support was added to GDK, we changed from recommending the use of multiple GitLab instances in the same GDK directory to each one isolated in a different one. So this has to be reflected into Geo instructions as well.

cc @dbalexandre @rspeicher @stanhu

doc/howto/postgresql_replication.md

 # PostgreSQL replication
+## Prerequisites
+
+This guide assumes you've already set up one GDK instance with GitLab EE to act
+as the **primary** Geo node in a `gdk-ee` folder. If not, follow the [Set up GDK
+guide](../set-up-gdk.md#gitlab-enterprise-edition) before continuing!
+
+## Configuring replication
+
 For Gitlab Geo, you will need a master/slave database replication defined.
 There are a few extra steps to follow:
-You must start with a clean postgres setup, (jump to next if you are installing
-everything from scratch):
+In your primary instance (`gdk-ee`) you need to prepare the database for
+replication. This requires the PostgreSQL server to be running, so we'll start
+the server, perform the change (via a `make` task), and then kill and restart
+the server to pick up the change:
 ```
-rm -rf postgresql
-make postgresql
-```
-Initialize a slave database and setup replication:
-```
+cd gdk-ee
+# terminal window 1:
+foreman start postgresql
+
+# terminal window 2:
+make postgresql-replication-primary
 # terminal window 1:
-make postgresql-replication/cluster
+# stop foreman by hitting Ctrl-C, then restart it:
 foreman start postgresql
+```
+
+Because we'll be replicating the primary database to the secondary, we need to
+remove the secondary's PostgreSQL data folder:
+```
 # terminal window 2:
-make postgresql-replication/role
-make postgresql-replication/backup
-# go back to terminal window 1 and stop foreman by hitting "CTRL-C"
+cd gdk-geo
+rm -r postgresql
+```
+
+Now we need to add a symbolic link to the primary instance's data folder:
+
+```
+# From the gdk-geo folder:
+ln -s ../gdk-ee/postgresql postgresql-primary
+```
+
+Initialize a slave database and setup replication:
 ```
+# terminal window 2:
+make postgresql-replication-secondary
+```
+
+Now you can go back to **terminal window 1** and stop `foreman` by hitting
+<kbd>Ctrl-C</kbd>.
+
+## Next steps
+
+Continue to the [Post-installation
+section](../set-up-gdk.md#post-installation) of the setup guide.