Source is not automatically set to hostname on legacy source-based metrics

[BRMatt]

At the moment if write_to_legacy is enabled and source is not specified in the config file metrics will be sent to the /metrics endpoint with a null source, which would break existing spaces that rely on this being set.

It seems the sink in the upstream sink defaults the source to socket.gethostname():

statsite/sinks/librato.py

Lines 115 to 120 in 76a3685

	if config.has_option(sect, 'source'):
	self.source = config.get(sect, 'source')
	else:
	self.source = socket.gethostname()