Explicit task parameters

Suggestion:

taskA:
    action: my_act p1={}
    on-success:
        -taskB(msg="yes")
    on-failure:
        - taskB(msg="no")

So the idea is to short-circuit publishing mechanism that we have now that can be used via "publish" clause. It means we can avoid using "publish" and specify explicit variables that will go into inbound context of a downstream task.

------ Example ------

tasks:

    task1:
        publish:
            message: My default message

    task2:
        on-error:
            - notify # In this case "notify" will use "My default message"
        on-success:
            - notify(message='My branch specific message') # "notify" won't use the default message.

    notify:
        action: send_message msg=<% $.message %>

Notes:

* Those explicit parameters essentially is a shortcut for publishing with the only difference that it publishes variables for one specific downstream task. More precisely, a branch that the downstream task belongs to.

[renat: Everyone at the PTG agreed with the proposed semantics]