API requires AGDC_API_CONFIG to be set, can't use default path

[andrewdhicks]

The API Config class uses the AGDC_API_CONFIG environment variable. If the variable is not set, os.environ throws a KeyError exception which is not handled. See https://github.com/GeoscienceAustralia/agdc/blob/develop/api/source/main/python/datacube/config.py#L49