Deployment update with External Ceph cluster fails with 0 objects found in container: overcloud_ceph_ansible_fetch_dir in mistral

Issue

When doing a stack update on a Red Hat OpenStack Platform 13 deployment which is configured to use an external ceph cluster via environments/ceph-ansible/ceph-ansible-external.yaml, the stack update fails with:

2018-11-07 15:59:38Z [overcloud-AllNodesDeploySteps-orpuuizd5xal.WorkflowTasks_Step2_Execution]: CREATE_FAILED  resources.WorkflowTasks_Step2_Execution: ERROR
2018-11-07 15:59:38Z [overcloud-AllNodesDeploySteps-orpuuizd5xal]: UPDATE_FAILED  Resource CREATE failed: resources.WorkflowTasks_Step2_Execution: ERROR
2018-11-07 15:59:39Z [AllNodesDeploySteps]: UPDATE_FAILED  resources.AllNodesDeploySteps: Resource CREATE failed: resources.WorkflowTasks_Step2_Execution: ERROR
2018-11-07 15:59:39Z [overcloud]: UPDATE_FAILED  Resource UPDATE failed: resources.AllNodesDeploySteps: Resource CREATE failed: resources.WorkflowTasks_Step2_Execution: ERROR

 Stack overcloud UPDATE_FAILED

overcloud.AllNodesDeploySteps.WorkflowTasks_Step2_Execution:
  resource_type: OS::TripleO::WorkflowSteps
  physical_resource_id: a56a3276-651b-4ba9-80ab-b201549f7101
  status: CREATE_FAILED
  status_reason: |
    resources.WorkflowTasks_Step2_Execution: ERROR
Heat Stack update failed.
Heat Stack update failed.

In mistral engine.log I see:

2018-11-07 10:59:37.243 14181 INFO workflow_trace [req-12be969e-4c9f-48c8-9521-b5f084e72445 c424161fc5c74afdab621c35c5a361f2 dbf7e0522d59438a936b0ccc0527b44a - default default] Workflow 'trip
leo.overcloud.workflow_tasks.step2' [RUNNING -> ERROR, msg=Failure caused by error in tasks: ceph_base_ansible_workflow

  ceph_base_ansible_workflow [task_ex_id=6391a9bb-2d50-417a-8743-580205f777f3] -> Failure caused by error in tasks: restore_fetch_directory

  restore_fetch_directory [task_ex_id=2c2c1c95-c418-4a0f-8214-45ae392c4b24] -> {msg: 0 objects found in container: overcloud_ceph_ansible_fetch_dir but one object was expected.}
    [action_ex_id=bb238f30-dfa9-4ef1-9919-e55cf74b6771, idx=0]: {u'msg': u'0 objects found in container: overcloud_ceph_ansible_fetch_dir but one object was expected.'}

    [wf_ex_id=f972e6fc-c93d-499d-ac97-bb0975200748, idx=0]: Failure caused by error in tasks: restore_fetch_directory

  restore_fetch_directory [task_ex_id=2c2c1c95-c418-4a0f-8214-45ae392c4b24] -> {msg: 0 objects found in container: overcloud_ceph_ansible_fetch_dir but one object was expected.}
    [action_ex_id=bb238f30-dfa9-4ef1-9919-e55cf74b6771, idx=0]: {u'msg': u'0 objects found in container: overcloud_ceph_ansible_fetch_dir but one object was expected.'}

] (execution_id=a56a3276-651b-4ba9-80ab-b201549f7101)