Fix config loading for Debian package

CONFIG_FILENAME in uwsgi.ini was copy-pasted from uffd without adding
support for it in uffd-nginxauth. However, the way it is used for uffd
does not work anyway, since it is appended to the app's instance_path
(it sets the filename not the path!). For some reason linking the config
file to "config.cfg" in the instance folder, does not work either.

This change introduces the environment variable CONFIG_PATH. If set, it's
value is used as is for reading the config file.