Simplify cluster configuration

Upgrading (or refactoring, comparing) a cluster configuration is harder than it should be. These configurations are pretty verbose at the moment, as a result of

• "environment" variables ("mode", "engine", ...) that are being shared between plugins
  lookup for these variables could fall back to the cluster config when the plugin itself doesn't have the required property

• the need to generate unique ids for services that are started in the cluster
  this should always happen for services and references; the framework should take care of linking these to those in other clusters

Cluster configurations such as those generated by the template builder should contain as little information as possible, ideally only data that was explicitly entered by a user.