Log level change for zipkin-collector

I provided a new collector-cassandra.scala config file that sets the log level to INFO.
By default the log level was set to DEBUG which means that every span submitted to the collector was being logged.  This slows down the collector and generates a huge log.

I guess INFO is good enough. You get some feedback when the collector is started but you don't get lots of irrelevant log entries. Changing it to another log level is also straightforward. The level is defined in collector-cassandra.scala