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

That wasn't what I meant. It's nice to be able to re-use (or just read and understand) the Spring Boot logger config files as standalones. If it's not valid on its own without a `Log4JLoggingSystem` then it's less valuable IMO.