SPARK-2038: rename "conf" parameters in the saveAsHadoop functions with source-compatibility 

https://issues.apache.org/jira/browse/SPARK-2038

to differentiate with SparkConf object and at the same time keep the source level compatibility 
Might be a bit more clear to say:
```
// Rename this as hadoopConf internally to avoid shadowing (see SPARK-2038).
```