[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)
Ok fair enough

(@mengxr I wasn't suggesting enumerations, just a pattern match on the `Option[String]` value as per comment. Don't believe this adds more code or complexity, but no strong feelings either way)