[LIVY-651] Provide a approach to get "owner" inside spark application

[yantzu]

What changes were proposed in this pull request?

When impersonation is disabled, hadoop UGI user will always be livy server user, there is no way to get actually owner user in spark application.

This issue is to fix this problem by add a "spark.livy.owner" property to spark context. Spark application code can get this property if needed.

https://issues.apache.org/jira/browse/LIVY-651

How was this patch tested?

Update existing unit test, and test manually.