Revert "consumer: removes unused task"

This reverts commit 441e28b.

* We are correctly using the
  [TTLSecondsAfterFinished](https://kubernetes.io/docs/concepts/workloads/controllers/ttlafterfinished),
  and it really works, but the subtle detail is that RJC never
  finishes. One can see this by checking how many ready containers
  are inside the pod, Workflow Engine finishes and RJC still runs,
  see:
  ```console
  $ kubectl get pod \
            batch-serial-a0f3f1c4-161d-461c-94e0-c4dae1000b2f-nv8bb -o json | \
    jq '.status.containerStatuses[] | .name + " " + (.ready|tostring)'
      "job-controller true"
      "workflow-engine false"
  ```
  Because of this we need to manually delete the batch- containers
  with the task that was removed in the previous commit
  (closes #258).

reana_workflow_controller/consumer.py

@@ -14,7 +14,9 @@
 import uuid
 
 import requests
+from kubernetes.client.rest import ApiException
 from reana_commons.consumer import BaseConsumer
+from reana_commons.k8s.api_client import current_k8s_batchv1_api_client
 from reana_commons.k8s.secrets import REANAUserSecretsStore
 from reana_commons.utils import (calculate_file_access_time,
                                  calculate_hash_of_dir,
@@ -25,6 +27,7 @@
 
 from reana_workflow_controller.config import (PROGRESS_STATUSES,
                                               REANA_GITLAB_URL, REANA_URL)
+from reana_workflow_controller.errors import REANAWorkflowControllerError
 
 
 class JobStatusConsumer(BaseConsumer):
@@ -71,6 +74,10 @@ def _update_workflow_status(workflow_uuid, status, logs):
         .one_or_none()
     if workflow.git_ref:
         _update_commit_status(workflow, status)
+    alive_statuses = \
+        [WorkflowStatus.created, WorkflowStatus.running, WorkflowStatus.queued]
+    if status not in alive_statuses:
+        _delete_workflow_engine_pod(workflow_uuid)
 
 
 def _update_commit_status(workflow, status):
@@ -175,3 +182,21 @@ def _update_job_cache(msg):
     cached_job.result_path = msg['caching_info'].get('result_path')
     cached_job.workspace_hash = workspace_hash
     Session.add(cached_job)
+
+
+def _delete_workflow_engine_pod(workflow_uuid):
+    """Delete workflow engine pod."""
+    try:
+        jobs = current_k8s_batchv1_api_client.list_namespaced_job(
+            namespace='default',
+        )
+        for job in jobs.items:
+            if workflow_uuid in job.metadata.name:
+                current_k8s_batchv1_api_client.delete_namespaced_job(
+                    namespace='default',
+                    propagation_policy="Background",
+                    name=job.metadata.name)
+                break
+    except ApiException as e:
+        raise REANAWorkflowControllerError(
+            "Workflow engine pod cound not be deleted {}.".format(e))