make it easier to pass in arbitrary arbitrary akka config

With this change, devs can easily modify arbitrary akka properties by passing them in as `spark.akkaExtra.<akka>.<property>.<here>`.  Also, they can turn off the slf4j logger inside akka by passing in `spark.akka.useSlf4jLogger=false` (I found it to be a pain to try to configure akka logging AND also configure slf4j as well, just the stdout logger was good enough).

Eventually maybe there should be a cleaner way for devs to pass in arbitrary akka config, but at least this is a step forward.  I'm not intending this to a be a publicly documented thing, but I think it will be helpful for anyone that needs to debug akka.