Feature: Allow combining stack-env-settings and app-env-settings

Until now, all app settings could either be set in `node[stackname]['env']` or in `node[appname]['env']`, which lead to a lot of config duplication.

With this PR, `node[stackname]['env']` is always fetched, and then merged with 
`node[appname]['env']`. App-Specific settings always override stack-specific settings.