Woodpecker should respect 'fio' action's 'disk-devices' parameter regardless of configuration of 'test-devices'

[Przemyslaw Hausman]

Woodpecker should respect 'fio' action's 'disk-devices' parameter regardless of configuration of 'test-devices'.

Currently, if 'test-devices' is configured, woodpecker ignores 'fio' action's 'disk-devices' parameter. See the current implementation in on_fio_action():

```
if not self.ceph_client.pools_available and test_devices:
    event.params["disk-devices"] = (
        " ".join([str(d.location) for d in test_devices])
    )
```

It makes it impossible to specify exactly which disk-devices should be used for testing.

In my case, I'd like to run fio tests on ephemeral device, instead of on cinder volumes attached to the woodpecker VM. For that. I have created a flavor with an ephemeral disk and I'm using this flavor when deploying woodpecker units:

```
openstack flavor create \
  --vcpus 4 \
  --ram 8192 \
  --disk 20 \
  --ephemeral 10 \
  m1.large.ephemeral

juju deploy cs:~openstack-charmers-next/woodpecker \
  --num-units ${NUMBER_OF_UNITS} \
  --storage test-devices=cinder,3,10G \
  --constraints "cores=4 allocate-public-ip=true instance-type=m1.large.ephemeral" \
  --series bionic \
  woodpecker
```

Once I have woodpecker units deployed, I want to run fio action only on top of ephemeral devices:

```
juju run --application woodpecker sudo umount /mnt

juju run-action woodpecker/0 \
  fio operation=randwrite runtime=900 block-size=4k disk-devices=/dev/vdb
``

Currently, the above action is not run on a specified disk (disk-devices=/dev/vdb), as I would expect. Instead it is being run on vdc, vdd, and vde -- cinder volumes attached to the VM.

As a workaround, I have to first detach the juju storage from the woodpecker VMs:
```
juju detach-storage test-devices/{0..NUMBER_OF_DISKS}
```

[Przemyslaw Hausman (phausman)]

Action documentation is not in line with the code that implements it:

Action documentation:
```
fio:
  description: "Run the fio performance test"
  params:
    disk-devices:
      type: string
      description: "If unset, use the charm default rbd device in the ceph pool or the block devices provided using test-devices storage. If set run fio, against the set disk. Space delimited list of devices."
```

Code:
```
        # If storage binding provided then override disk-devices
        # unless the application is related to ceph.
        if not self.ceph_client.pools_available and test_devices:
            event.params["disk-devices"] = (
                " ".join([str(d.location) for d in test_devices])
            )
```