I do not want HEALTHCHECKS environment variable to be mandatory.
If list of healthchecks is not provided, it defaults to "discovery in current working directory".
See also `if not healthchecks: healthchecks = [os.path.abspath(os.getcwd())]` in main().

Moreover, I'd like to preserve the previous way to run the server, i.e. `python -m hospital.wsgi [healthcheck, ...]` where healthchecks are not passed as environment variable but as argument.