Add support for logging configuration file in segment-gatherer

[gerritholl]

Segment gatherer currently has a hardcoded logging configuration, logging everything to stderr or to a file with hardcoded internal log rotation:

pytroll-collectors/pytroll_collectors/logging.py

Lines 5 to 27 in f062370

	def setup_logging(opts, name):
	"""Set up logging."""
	handlers = []
	if opts.log:
	handlers.append(logging.handlers.TimedRotatingFileHandler(opts.log,
	"midnight",
	backupCount=7))
	handlers.append(logging.StreamHandler())
	if opts.verbose:
	loglevel = logging.DEBUG
	else:
	loglevel = logging.INFO
	for handler in handlers:
	handler.setFormatter(logging.Formatter("[%(levelname)s: %(asctime)s :"
	" %(name)s] %(message)s",
	'%Y-%m-%d %H:%M:%S'))
	handler.setLevel(loglevel)
	logging.getLogger('').setLevel(loglevel)
	logging.getLogger('').addHandler(handler)
	logging.getLogger("posttroll").setLevel(logging.INFO)
	return logging.getLogger(name)

Other pytroll packages, such as pytroll-watcher or trollflow2, can use an external logging configuration file logging.yaml. It would be nice if the segment gatherer could also use logging.yaml.