Redo Galera-Cluster-Connection-Routing-Tutorial.md, possibly others as well

Galera-Cluster-Connection-Routing-Tutorial.md is almost a copy-paste of MySQL-Replication-Read-Write-Splitting-Tutorial.md. There may be other tutorials that look similar, I haven't checked. Anyways, it's quite wasteful to have the same text in many documents and it makes updating common things laborous.

To fix this, there should be one "general setup" tutorial, which goes through the common things and maybe sets up a simple service etc. Other tutorials can then be based on this, with the text indicating that "starting from where we left in the basic tutorial... Change X to Y... A will happen instead of B..." and so on. This will save a lot of typing and make the additional tutorials much shorter.