Adds REST API endpoint for action execution

Deployment is replaced with a more generic concept called 'action'.
Action can be performed on any object of Object Model.
Actions are marked with 'Usage: Action' in MuranoPL code. They can have arguments.
List of available actions can be obtained from Object Model itself after initial deployment.
This commit adds ability to REST API invoke actions by providing its unique id (from OM) and parameters.
Also refactors API code to use tasks.

Change-Id: If21809340bb799af58a8d1a2d148e52565028970
Partially-Implements: blueprint application-actions