Cherry-pick #60 (LOG_LISTENER_CONN_SUCESS) into master

Here's the [original PR](https://github.com/graphite-project/carbon/pull/60) text from @mrtazz:

this patch adds a config option which can be added for the
cache/relay/aggregator instances to not log successful connections anymore.
For this the config option LOG_LISTENER_CONN_SUCCESS has to be set to False.