Allow each HiveContext to have distinct users that they will use to connect to the HiveMetastore

Currently the HiveContexts use the UserGroupInformation to determine which user to connect to the HiveMetastore.

This is either determined from an Environment variable, the user that is running the spark process, or a static user.

With Spark 1.6 there was the introduction of multiple HiveContext Sessions but each session uses the same HiveClient which uses the same user.

We would like access to specify which user to connect to the Hive Metastore for each HiveContext so that it will integrate better with our Authentication and Authorization mechanisms.