IMPALA-5162,IMPALA-5163: stress test support on secure clusters

This patch adds support for running the stress test
(concurrent_select.py) and loading nested data (load_nested.py) into a
Kerberized, SSL-enabled Impala cluster. It assumes the calling user
already has a valid Kerberos ticket. One way to do that is:

1. Get access to a keytab and krb5.config
2. Set KRB5_CONFIG and KRB5CCNAME appropriately
3. Run kinit(1)
4. Run load_nested.py and/or concurrent_select.py within this
   environment.

Because our Python clients already support Kerberos and SSL, we simply
need to make sure to use the correct options when calling the entry
points and initializing the clients:

Impala: Impyla
Hive: Impyla
HDFS: hdfs.ext.kerberos.KerberosClient

With this patch, I was able to manually do a short concurrent_select.py
run against a secure cluster without connection or auth errors, and I
was able to do the same with load_nested.py for a cluster that already
had TPC-H loaded.

Follow-ons for future cleanup work:

IMPALA-5263: support CA bundles when running stress test against SSL'd
             Impala

IMPALA-5264: fix InsecurePlatformWarning under stress test with SSL

Change-Id: I0daad57bb8ceeb5071b75125f11c1997ed7e0179
Reviewed-on: http://gerrit.cloudera.org:8080/6763
Reviewed-by: Matthew Mulder <mmulder@cloudera.com>
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: Impala Public Jenkins

testdata/bin/load_nested.py

@@ -298,6 +298,8 @@ if __name__ == "__main__":
   parser = ArgumentParser(formatter_class=ArgumentDefaultsHelpFormatter)
   cli_options.add_logging_options(parser)
   cli_options.add_cluster_options(parser)  # --cm-host and similar args added here
+  cli_options.add_kerberos_options(parser)
+  cli_options.add_ssl_options(parser)
 
   parser.add_argument("-s", "--source-db", default="tpch_parquet")
   parser.add_argument("-t", "--target-db", default="tpch_nested_parquet")