IMPALA-13123: Add option to run tests with Python 3

This introduces the IMPALA_USE_PYTHON3_TESTS environment variable
to select whether to run tests using the toolchain Python 3.
This is an experimental option, so it defaults to false,
continuing to run tests with Python 2.

This fixes a first batch of Python 2 vs 3 issues:
 - Deciding whether to open a file in bytes mode or text mode
 - Adapting to APIs that operate on bytes in Python 3 (e.g. codecs)
 - Eliminating 'basestring' and 'unicode' locations in tests/ by using
   the recommendations from future
   ( https://python-future.org/compatible_idioms.html#basestring and
     https://python-future.org/compatible_idioms.html#unicode )
 - Uses impala-python3 for bin/start-impala-cluster.py

All fixes leave the Python 2 path working normally.

Testing:
 - Ran an exhaustive run with Python 2 to verify nothing broke
 - Verified that the new environment variable works and that
   it uses Python 3 from the toolchain when specified

Change-Id: I177d9b8eae9b99ba536ca5c598b07208c3887f8c
Reviewed-on: http://gerrit.cloudera.org:8080/21474
Reviewed-by: Michael Smith <michael.smith@cloudera.com>
Reviewed-by: Riza Suminto <riza.suminto@cloudera.com>
Tested-by: Joe McDonnell <joemcdonnell@cloudera.com>

lib/python/impala_py_lib/helpers.py

@@ -37,7 +37,8 @@ def exec_local_command(cmd):
   Return:
     STDOUT
   """
-  proc = subprocess.Popen(cmd.split(), stdout=subprocess.PIPE, stderr=subprocess.PIPE)
+  proc = subprocess.Popen(cmd.split(), stdout=subprocess.PIPE, stderr=subprocess.PIPE,
+                          universal_newlines=True)
   output, error = proc.communicate()
   retcode = proc.poll()
   if retcode: