No-op task

We should be able not to specify either 'action' or 'workflow' for a task. In this case the task, in fact, will be just a decision making point. For example, once we get 'join' done we can have multiple inbound tasks joined on another tasks where we don't need to call any actions/workflow but rather just need to understand what to do next.

Suggested syntax:

...
tasks:
  my_no_op_task:
    on-complete:
      - my_next_task: $.my_ctx_var = 10