[SPARK-9491] Avoid fetching HBase tokens when not needed.

Look at HBase's configuration to make sure it's configured for
Kerberos. If the HBase configuration is missing, or if HBase is
configured for non-kerberos authentication, then skip getting
tokens.

Reference: http://hbase.apache.org/book.html#security.prerequisites

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #7810 from vanzin/SPARK-9491 and squashes the following commits:

a57c776 [Marcelo Vanzin] [SPARK-9491] Avoid fetching HBase tokens when not needed.