Non-distributable actions

We need to introduce a notion of non-distributable actions. The rationale is simple: many actions (like echo, and e.g. some math actions we'll have in the future) are pointless to distribute. Distribution overhead for them would be much higher than their execution time.

Here's what I think we could do:
* Add method "is_distributable" into Action interface
* Add configuration property to enforce all actions to be distributable/non-distributable (the second makes sense if we want to test workflows consisting of even simple actions at scale)