[SPARK-1395] Fix "local:" URI support in Yarn mode (again).

Just update some calls to call methods that properly
handle "local:" URIs. Aside from those, two changes:

- the SPARK_JAR env variable needs to be propagated to
  remote processes when the spark jar is "local:",
  otherwise those processes won't be able to find it
  in their PWD (since the file won't be distributed)

- I removed the hacky way that log4j configuration was
  being propagated to handle the "local:" case. It's
  much more cleanly (and generically) handled by using
  spark-submit arguments (--files to upload a config
  file, or setting spark.executor.extraJavaOptions to
  pass JVM arguments and use a local file).