Lower logger level in OF1.0 Discovery Service

Lower logger level in OF1.0 Discovery Service

- Discovery Service has a function to attempt to snoop
  non controller generated or malformed LLDP frames.
  The whole function block is in a try catch block,
  as exceptions are well expected.
  In case an exception is hit though, the function logs
  it as a warn.
- Change the log level to debug, do not print a stack trace,
  print the source MAC and the port on which it was received.

Change-Id: I82b459b16356b0a57934b3c26c993d20a2b53dc2
Signed-off-by: Alessandro Boch <aboch@cisco.com>