'__call__(config) -> str
Check whether the configuration point is set correctly.
Arguments:
config(dict): Dictionary of all configuration points
Returns:
A tuple (result, message) where result is whether the
option is correctly configured, and message is an optional
message describing the error.'

def __call__(self, config):

    if (not self.relevant(config)):
        return (True, '')
    return self.check(config.get(self.name, None))
