tests: don't run functional tests in parallel

kafka doesn't play well with //, each times a new partition/topic is
created, kafka rebalances things across brokers and messages are not
delivered at the expected time.

So this change disable the parallelizism for functional tests for kafka
only.

Change-Id: Ia497334efcd4685850ac1529a7fc38073dae7cd1

.testr.conf

@@ -2,3 +2,4 @@
 test_command=OS_STDOUT_CAPTURE=${OS_STDOUT_CAPTURE:-1} OS_STDERR_CAPTURE=${OS_STDERR_CAPTURE:-1} OS_TEST_TIMEOUT=${OS_TEST_TIMEOUT:-60} ${PYTHON:-python} -m subunit.run discover -t ./ . $LISTOPT $IDOPTION
 test_id_option=--load-list $IDFILE
 test_list_option=--list
+group_regex=${OS_GROUP_REGEX}

tox.ini

@@ -55,6 +55,7 @@ commands = pifpaf run rabbitmq --  python setup.py testr --slowest --testr-args=
 setenv =
     {[testenv]setenv}
     TRANSPORT_DRIVER=kafka
+    OS_GROUP_REGEX=oslo_messaging.tests.functional
     kafka-python>=1.3.1
 commands = {toxinidir}/setup-test-env-kafka.sh python setup.py testr --slowest --testr-args='{posargs:oslo_messaging.tests.functional}'