Resource sentinel nodes inside generators use incorrect task id's

[mchalek]

We have found that under the following conditions, an error occurs:

• A DAG uses a generator with at least two elements
• The generator node has downstream dependencies
• A resource is created inside the generator workflow
• At least one node in the generator workflow both requires the resource and has no downstream dependencies that do not require the resource.

Under these conditions, boundary-layer by default inserts a sentinel node downstream of the node(s) matching these conditions, in order to propagate errors past the resource-destroy step. However, inside the generator, the sentinel node that is created does not have <<item_name>> appended to its task_id. This causes errors when that node is connected to the generator's downstream dependencies, because the Airflow set_upstream() method will be called repeatedly (once per generator element) for the (sentinel_node, generator_downstream_dependency) pairs. Airflow is proactive about alerting on this, which is nice (although in theory it's not really a problem...).

Anyway this is a bug that we should fix.