Workaround kubernetes python client deadlock issue

The kubernetes python client has a bug [1] which results in frequent
deadlocks while being cleaned up, which causes armada to hang at the
end of execution.

This patchset works around that issue by mocking out the associated
thread pools, since they are only needed for async kubernetes api
calls, which armada does not use.

[1]: kubernetes-client/python#411

Change-Id: I71fbfbe355347ae2ddd02ffd26d881368320246b

armada/handlers/k8s.py

@@ -17,7 +17,9 @@
 from kubernetes import client
 from kubernetes import config
 from kubernetes import watch
+from kubernetes.client import api_client
 from kubernetes.client.rest import ApiException
+from unittest.mock import Mock
 from oslo_config import cfg
 from oslo_log import log as logging
 
@@ -27,6 +29,12 @@
 CONF = cfg.CONF
 LOG = logging.getLogger(__name__)
 
+# TODO: Remove after this bug is fixed and we have uplifted to a fixed version:
+#       https://github.com/kubernetes-client/python/issues/411
+# Avoid creating thread pools in kubernetes api_client.
+_dummy_pool = Mock()
+api_client.ThreadPool = lambda *args, **kwargs: _dummy_pool
+
 
 class K8s(object):
     '''