Working on Data Flow (step 3)

* Refactored tests hierarcy (moved get_resource to the base class)
* Fixed logic in abstract engine (transaction scoope was wrong)
* Added stub methods for context manipulations (not everywhere yet)
* Refactored actions
* Created ECHO action to be able to emulate required output
* Added simple data flow test (two dependent tasks)

TODO:
* More tests
* Refactor actions according to the latest discussions

Change-Id: I5ba5e330110889014eeca53501ddae54dc9a1236