Use .script for the .app.src file for backend

Rebar will invoke the .app.src.script file to produce the .app.src
file used to configure the application.

make ct passes
Permit customization of application environment

Specifically, make it easy to customize the environment of a specific
application at build time, without having to specify all settings in a
single file. While rebar app_vars_file may work in a similar fashion,
having a minimal file that overrides only the environment settings is
ideal for the purpose of customizing the application build. Besides,
there seems to be a lack of a good use of app_vars_file.

make ct passes