[FLINK-1097] Fixed multiple slf4j bindings when using Hadoop2

I excluded all slf4j-log4j12 transitive dependencies to get rid off the warning that slf4j finds multiple slf4j bindings in the classpath. Additionally, I added log4j-over-slf4j and excluded all log4j transitive dependencies so that slf4j handles also legacy log4j logging messages from dependencies.

Author: Till Rohrmann <trohrmann@apache.org>

Closes #121 from tillrohrmann/FLINK-1097 and squashes the following commits:

b9f76d5 [Till Rohrmann] Excluded slf4j-log4j12 as transitive dependency in order to not have multiple slf4j bindings in the class path. Added log4j-overslf4j and excluded log4j transitive dependencies to collect log4j outputs.