Find k3s device from volume ID instead of trusting openstack

[AlexCK-STFC]

openstack_compute_volume_attach_v2.data_volume_attach.device is often wrong at provision time and should not be trusted

Instead, the same approach used in azimuth-images should be used, finding the device from the volume ID.

This avoids errors like:

TASK [azimuth_cloud.azimuth_ops.k3s : Fail if k3s storage device is missing] **************************************************************************************************************************
fatal: [azimuth-stfc-dev]: FAILED! => {"changed": false, "msg": "K3s storage device not found at /dev/vdb"}

when the hypervisor chooses the mount the volume in a different mount than Terraform/OpenStack is expecting.

[AlexCK-STFC]

I wasn't sure why k3s_storage_device was set in /defaults/main.yml so removed it; it should be set by terraform, not sure of a time this default would be used.

[m-bull]

Thanks for the patch - its a good idea and we should've done it this way from the start.

General comment on the implementation - I would value the view of @JohnGarbutt and @sd109 here too.

[m-bull]

Thinking more about this, I think we want the actual /dev/disk/by-id path in /etc/fstab (which happens in the mount filesytem task below), otherwise I think we could end up in a situation like this:

1. Attach a volume, it is /dev/disk/by-id/SOME-UUID => /dev/vdb
2. Ansible here adds /dev/vdb to /etc/fstab
3. Reboot the machine, libvirt shuffles the cards and now /dev/disk/by-id/SOME-UUID => /dev/vdc
4. Machine never boots because its missing a device it expects to see in /etc/fstab.
5. (Machine miraculously boots, but then we run the ansible again and we have /dev/vdb and /dev/vdc in /etc/fstab.

I think this task should look more like:

    - name: Set volume block device name
      ansible.builtin.set_fact:
        k3s_storage_device: >-
          {{
            candidate_device_paths_stat.results |
              selectattr("stat.exists") |
              map(attribute = "stat.path") |
              first |
              default("") |
             }}

then k3s_storage_device will end up with the /dev/disk/by-id path, and we should be fully immune from libvirt shuffling.

If you go this way, then you can set when: k3s_storage_device == "" in the next task, which is somewhat cleaner I think.

[m-bull]

Note to self - if we move to this method, we should do it in a release where we change the base image, because that way we don't have to worry about the oldstyle (/dev/vdb) and newstyle (/dev/disk/by-id/) coexisting in /etc/fstab, because we'll be rolling the image and will have a fresh fstab.

[AlexCK-STFC]

I don't have enough time to adopt this change myself right now, can this PR be taken over @m-bull?