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
`yaml` probably does the crap thing where it sometimes bytes and sometimes unicode, but that's true everywhere. So no need to bother with conversion.

The code that *utilizes* this will want to do `key.encode("ascii")` and likewise for the value.