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

@@ -3,24 +3,41 @@
 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:
 ```
+cd gdk-ee
+# terminal window 1:
+foreman start postgresql
+
+# terminal window 2:
+make postgresql-replication-primary
+
+# go back to terminal window 1 stop foreman by hitting "CTRL-C" and execute it again with:
+foreman start postgresql
+```
+
+You must start with a clean postgres setup on the secondary node:
+
+```
+# terminal window 2:
+cd ../gdk-geo
 rm -rf postgresql
-make postgresql
 ```
-Initialize a slave database and setup replication:
+You need to setup a symbolic link to the `postgresql` folder from the
+primary instance (`gdk-ee`):
 ```
-# terminal window 1:
-make postgresql-replication/cluster
-foreman start postgresql
-# terminal window 2:
-make postgresql-replication/role
-make postgresql-replication/backup
-# go back to terminal window 1 and stop foreman by hitting "CTRL-C"
+# you must be in `gdk-geo` folder
+ln -s ../gdk-ee/postgresql postgresql-primary
+```
+Initialize a slave database and setup replication:
+```
+# terminal window 2:
+make postgresql-replication-secondary
+# go back to terminal window 1 foreman by hitting "CTRL-C"
 ```