spring-boot-1612: make console and file output configurable

1 disable the file output unless LOG_FILE is set
2 make the console optional

fixes: gh-1612

Do we really need to use the Java API for the loggers here? I was hoping this would be in a config file. What happens if the user sets additional logger levels later, do they inherit from their parent or something? If so why do we need to iterate here (can't you just set the handler for the root logger).