Ansible: Debug and iteration over dicts fails in ansible playbook

Ansible version:
ansible 2.6.2
python version
2.7.5

I have a debug output from an ansible playbook which gives this:

"msg": {
    "changed": false,
    "failed": false,
    "guest_disk_facts": {
        "0": {
            "backing_datastore": "datastore1",
            "backing_disk_mode": "persistent",
            "backing_eagerlyscrub": false,
            "backing_filename": "[datastore1] test-1.vmdk",
            "backing_thinprovisioned": false,
            "backing_uuid": "6000C290-3e84-38aa-4e56-fbaba882190c",
            "backing_writethrough": false,
            "capacity_in_bytes": 53687091200,
            "capacity_in_kb": 52428800,
            "controller_key": 1000,
            "key": 2000,
            "label": "Hard disk 1",
            "summary": "52,428,800 KB",
            "unit_number": 0
        },
        "1": {
            "backing_datastore": "datastore2",
            "backing_disk_mode": "persistent",
            "backing_eagerlyscrub": true,
            "backing_filename": "[datastore2] test-1.vmdk",
            "backing_thinprovisioned": false,
            "backing_uuid": "6000C297-5d68-8010-7b09-75cf83e30801",
            "backing_writethrough": false,
            "capacity_in_bytes": 10737418240,
            "capacity_in_kb": 10485760,
            "controller_key": 1000,
            "key": 2001,
            "label": "Hard disk 2",
            "summary": "10,485,760 KB",
            "unit_number": 1
        },
        "2": {
            "backing_datastore": "datastore5",
            "backing_disk_mode": "persistent",
            "backing_eagerlyscrub": false,
            "backing_filename": "[datastore5] test-1.vmdk",
            "backing_thinprovisioned": false,
            "backing_uuid": "6000C292-1f84-3549-c198-d660e6955eda",
            "backing_writethrough": false,
            "capacity_in_bytes": 150323855360,
            "capacity_in_kb": 146800640,
            "controller_key": 1000,
            "key": 2002,
            "label": "Hard disk 3",
            "summary": "146,800,640 KB",
            "unit_number": 2
        }
    }
}

}

The sample tasks in playbook is as below which gives this output:

 - name: Debug disk facts
  debug: msg={{ output }}

How can i iterate over the arrays to print just the "backing_datastore" information, so i manage to register it in a variable or stdout?

I have tried to do this and it fails. yet i need values from all arrays

debug: msg={{ output.guest_disk_facts.0  }}

fatal: [localhost]: FAILED! => {"msg": "The task includes an option with an undefined variable. The error was: dict object has no element 0\n\nThe error appears to have been in ...