[SPARK-2098] All Spark processes should support spark-defaults.conf, config file

This is another implementation about #1256
cc @andrewor14 @vanzin
This map and filter is duplicate. We can rewrite this to avoid that:
```
configFile
  .orElse(...)
  .filter(_.isFile)
  .map(_.getAbsolutePath)
  .orNull
```
Then you don't need to do the same map and filter up there in L1402