Fetching delegation tokens for Hive fails when using proxy users

If you use spark-submit's proxy user support, the code that fetches delegation tokens for the Hive Metastore fails. It seems like the Hive library tries to connect to the Metastore as the proxy user, and it doesn't have a Kerberos TGT for that user, so it fails.

I don't know whether the same issue exists in the HBase code, but I'll make a similar change so that both behave similarly.