Add environment variables YAML config for Applications

fixes #569 
add optional "environment" key to application YAML config
a dictionary of environment variable mappings to be made
available to the application's container
Or if you prefer to do duck typing, you could assign this iterator to a variable surrounded by the try: except. I'd prefer that than having the try: except around the entire loop....what if AttributeError was raised from one of the operations in the for loop ?