[SPARK-2550][MLLIB][APACHE SPARK] Support regularization and intercept in pyspark's linear methods

Related to Jira Issue: [SPARK-2550](https://issues.apache.org/jira/browse/SPARK-2550?jql=project%20%3D%20SPARK%20AND%20resolution%20%3D%20Unresolved%20AND%20priority%20%3D%20Major%20ORDER%20BY%20key%20DESC)
Py4j will pass through Python `None` as null (at least it should, if I recall), so on the Java side you can wrap that in an `Option` instead of making it "none".

So you could do:
```
Option(regType) match {
  case Some("l1") => ...
  case Some("l2") => ...
  case None => ...
}
```