Let SparkInterpreter use Credentials

Currently SparkInterpreter doesn't have any user impersonation abilities and sparkContext gets assigned the username of machine where it's run. However user's would like to impersonate sparkInterpreter, especially in shared mode. That would enable them for example reading and writing to hdfs with their desired usernames. For that we need:
1. establish impersonation routine
2. pass corresponding credentials for sparkInterpreter from credentials menu