Change router.register log message to Debug

I think notifying logging of a router register should be a debug level message not info.

* Frequent router register log messages make it very hard to follow router logs when diagnosing an issue.
* We are sending all our CF logs to our Enterprise Splunk.  Splunk charges by the MB sent to splunk.  So par router.register messages are accounting for 75% of our log messages.
symmetry between router.unregister and router.register log msgs

Signed-off-by: Glenn Oppegard <goppegard@pivotallabs.com>