BP mistral-actions-design (add new actions package)

* Base class for actions (Action)
* Class for representing action namespaces (NameSpace)
* Module action_factory.py for registering action classes, accessing
  them and creating action instances
* Unit tests for action_factory.py
* Standard actions (echo, http, email)

TODO:
* Ad-hoc per-workflow namespaces and actions
* Mistral HTTP standard action
* Refactoring executor to use new action infrastructure
* Remove old action infrastructure
* Action plugin architecture

Partially implements: blueprint mistral-actions-design
Partially implements: blueprint mistral-change-rest-api-to-http

Change-Id: I8e79b1f8173809584f0156e666e1675f5bd7bb37
BP mistral-actions-design (action creation)

* Method create_action in action_factory.py
* Method get_action_class in action_factory.py
* Additional tests for action_factory.py
* Removing namespace and action name params from action constructors

TODO:
* Ad-hoc per-workflow namespaces and actions
* Mistral HTTP standard action
* Refactoring executor to use new action infrastructure
* Remove old action infrastructure
* Action plugin architecture

Partially implements: blueprint mistral-actions-design
Partially implements: blueprint mistral-change-rest-api-to-http

Change-Id: Ia90c66e9e06a577d06d3db687098cdb6d741afaa