BP mistral-actions-design (Mistral HTTP action)

* Created MistralHTTPAction
* More unit tests
* Refactoring

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

Partially implements: blueprint mistral-actions-design

Change-Id: I6856d6587243872a67fa42efff0bf3f88ec19d82