Rename action_identifier

[Taucher2003]

We want to use this identifier not only for actions, but also for taurus and draco.

A new name could be something like definition_source

[raphael-goetz]

Done in: #206 but now I got a very small problem inside the node function.

The action identifier there is optional, now that every service sends an identifier, the field will never be optional. I used the filed to check if its a runtime function (if the field is not present) or a action function (the field would be present)

Now I can't differentiate between a runtime function and an action function easily. This is not really a problem, I just need to check if the function is present in taurus before execution each function. The optional just was faster for taurus to check performance wise.