Environment to disable workflow_tasks

Adds an optional environment that can be used to disable workflow_tasks
by mapping OS::Mistral resources to None. This environment can be used
by CI so that the updated ceph jobs
(I757be222143e41392b474d6b20c7a7b7df4537de) can be converted to
external_deploy_tasks, but the existing workflow_tasks can be
temporarily disabled until they are removed from the templates.

Change-Id: Ib39313712ea03e5562e7b19875c178e9c8dfef54

environments/disable-workflow-tasks.yaml

@@ -0,0 +1,6 @@
+# This environment can be used to optionally disable the mistral workflow_tasks
+# by mapping OS::Mistral resources to None.
+
+resource_registry:
+  OS::Mistral::Workflow: OS::Heat::None
+  OS::Mistral::ExternalResource: OS::Heat::None