Comment 1 for bug 1785597

When calling the action used to get the blacklisted_hostnames argument, this is what we get: (undercloud) [zuul@subnode-0 ~]$ openstack action execution run heat.stacks_output_show '{"stack_id" : "overcloud", "output_key" : "BlacklistedHostnames" }'
{"result": {"output": {"output_value": [""], "output_key": "BlacklistedHostnames", "description": "List of blacklisted hostnames"}}}

So, self.blacklisted_hostnames is by default [""] if not BlacklistedHostnames were defined for the role.