Unable to customize APP_MODULE variable in gunicorn template

[derektamsen]

Some python projects do not use the application module name of app:app that is hard coded into the template. Should add a variable that defaults to app:app but allows people to pass in a different module name if needed.

Ex: https://github.com/nedap/puppetboard#nginx-gunicorn which uses puppetboard.app:app

Per the gunicorn docs (http://gunicorn-docs.readthedocs.org/en/latest/run.html), this follows the format of $(MODULE_NAME):$(VARIABLE_NAME).