Working on Data Flow (step 1)

* Refactored API layer so that we can work with 'context' as with a
 json object in underlying layers (DB, engine, etc.) rather than
 a string
* Added "context" parameter in all required places
* Added necessary Data Flow related properties to DB models
* Refactored and fixed a series of tests
* Minor formatting changes

TODO:
* Calculation of task incoming context
* Abstract interface for expression evaluator
* Data Flow related tests

Partially implements blueprint: mistral-dataflow

Change-Id: Ie7f94d79265e9861f7ad15c76ff6d788ec62b683
Working on Data Flow (step 2)

* Added ExpressionEvaluator interface and YAQL implementation
* Removed yaql_utils, evaluator is now a more flexible replacement
* Fixed the places where yaql_utils was used
* Adjusted test cases

Change-Id: I7de70e678c211df34788763047b8361de0d3902a