[SPARK-13478][YARN] Use real user when fetching delegation tokens.

The Hive client library is not smart enough to notice that the current
user is a proxy user; so when using a proxy user, it fails to fetch
delegation tokens from the metastore because of a missing kerberos
TGT for the current user.

To fix it, just run the code that fetches the delegation token as the
real logged in user.

Tested on a kerberos cluster both submitting normally and with a proxy
user; Hive and HBase tokens are retrieved correctly in both cases.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #11358 from vanzin/SPARK-13478.