Implementing task execution infrastructure

* Implemented action calls
* Implemented subworkflow calls
* Adding executor interface and default implementation
* Unit tests
* Small refactoring in workflow handlers
* Creating all necessary RPC infrastructure
* Refactoring launch script
* Added __repr__() implementation to MistralContext
* Small fixes in old infrastructure

Change-Id: I134ea526c295ca9bda7214c5403a41966062ff79
Merge "Implementing task execution infrastructure"