try to use cwd() + gunicorn.conf.py as default config if no file is specified . see #52
[gunicorn] added ```gunicorn.conf.py``` as default in case of no -c is specified
prevent call get_default_config_file if args.config exists
Added coverage for get_default_config_file, also a test for default config overrides
Merge pull request #552 from niedbalski/master