Change logging backend of SLF4j from Logback to Log4j

... for better compatibility with YARN.

We'll still ship the Logback configuration file and pass them their path to the JVM so that
users can use Flink with Logback as well.

Please do not merge this in the next 12 hours. I would like to test the changes on my laptop @ work. I have a Hortonworks sandbox there to test it.
Thank you. I addressed both comments.