Make testing scripts python3.x compatible

Update run_tests/*.py to use six based isomorphisms and print function
from __future__ module.
diff --git a/tools/run_tests/run_interop_tests.py b/tools/run_tests/run_interop_tests.py
index 161fa81..ace142e 100755
--- a/tools/run_tests/run_interop_tests.py
+++ b/tools/run_tests/run_interop_tests.py
@@ -44,6 +44,7 @@
 import tempfile
 import time
 import uuid
+import six
 
 import python_utils.dockerjob as dockerjob
 import python_utils.jobset as jobset
@@ -885,7 +886,7 @@
 
 languages = set(_LANGUAGES[l]
                 for l in itertools.chain.from_iterable(
-                    _LANGUAGES.iterkeys() if x == 'all' else [x]
+                    six.iterkeys(_LANGUAGES) if x == 'all' else [x]
                     for x in args.language))
 
 languages_http2_badserver_interop = set()
@@ -925,7 +926,7 @@
     else:
       jobset.message('FAILED', 'Failed to build interop docker images.',
                      do_newline=True)
-      for image in docker_images.itervalues():
+      for image in six.itervalues(docker_images):
         dockerjob.remove_image(image, skip_nonexistent=True)
       sys.exit(1)
 
@@ -1053,7 +1054,7 @@
 
   if not jobs:
     print('No jobs to run.')
-    for image in docker_images.itervalues():
+    for image in six.itervalues(docker_images):
       dockerjob.remove_image(image, skip_nonexistent=True)
     sys.exit(1)
 
@@ -1093,9 +1094,9 @@
     if not job.is_running():
       print('Server "%s" has exited prematurely.' % server)
 
-  dockerjob.finish_jobs([j for j in server_jobs.itervalues()])
+  dockerjob.finish_jobs([j for j in six.itervalues(server_jobs)])
 
-  for image in docker_images.itervalues():
+  for image in six.itervalues(docker_images):
     if not args.manual_run:
       print('Removing docker image %s' % image)
       dockerjob.remove_image(image)