Turn off checkstyle for protobuf generated code.
Turn off checkstyle for protobuf generated code.

- Use protobuf generated comment to turn off checkstyle,
  so that we don't have to keep updateing exclusion rules.

Change-Id: Ic10d5576b104d67c2b83114741ced0768d09fca2

opennetworkinglab/spring-open,opennetworkinglab/spring-open,opennetworkinglab/spring-open,opennetworkinglab/spring-open,opennetworkinglab/spring-open,opennetworkinglab/spring-open