Configuration property to deny "merge" behavior

When a workflow has tasks with multiple inbound connections there are two patterns that Mistral supports:

* "join" - the task having a "join" property will run only once when its predesessors are completed (or part of them depending on the "join" value)
* "merge" - the task will run N number of times where N is the number of predecessors. Note that in this case we don't set any specific properties for a task, there's no "merge" property that should be explicitly configured.

This blueprints is about adding a config property to deny "merge" behavior completely. Some people tend to forget to specify "join" and it ends up behaving unexpectedly for them. For some installations, it's preferred to just disable it completely. If it's disabled Mistral can just fail validation check of a workflow text if it has at least one task with multiple inbound connections but w/o the "join" property set.