Establish convention for denoting Puppet resource relationships

[matze]

Resource order in Puppet is ​declared in various fashion. We have learned that the various ways to achieve the same goal (irregardless whether that is a static or dynamic relationship) can lead to quite some confusion (see in particular the ​code-review for ticket #4073). For the sake of better insight and more clean code, we should thus establish a convention that focuses on a single way to denote relationships, ideally one that works in every situation -- in order to avoid conditionals in both code and authoring.