purge_cluster: fix bug when building device list

purge_cluster: fix bug when building device list

there is some leftover on devices when purging osds because of a invalid
device list construction.

typical error:
```
changed: [osd3] => (item=/dev/sda sda1) => {
    "changed": true,
    "cmd": "# if the disk passed is a raw device AND the boot system disk\n if parted -s \"/dev/sda sda1\" print | grep -sq boot; then\n echo \"Looks like /dev/sda sda1 has a boot partition,\"\n echo \"if you want to delete specific partitions point to the partition instead of the raw device\"\n echo \"Do not use your system disk!\"\n exit 1\n fi\n echo sgdisk -Z \"/dev/sda sda1\"\n echo dd if=/dev/zero of=\"/dev/sda sda1\" bs=1M count=200\n echo udevadm settle --timeout=600",
    "delta": "0:00:00.015188",
    "end": "2018-05-16 12:41:40.408597",
    "item": "/dev/sda sda1",
    "rc": 0,
    "start": "2018-05-16 12:41:40.393409"
}

STDOUT:

sgdisk -Z /dev/sda sda1
dd if=/dev/zero of=/dev/sda sda1 bs=1M count=200
udevadm settle --timeout=600

STDERR:

Error: Could not stat device /dev/sda sda1 - No such file or directory.
```

the devices list in the task `resolve parent device` isn't built
properly because the command used to resolve the parent device doesn't
return the expected output

eg:

```
changed: [osd3] => (item=/dev/sda1) => {
    "changed": true,
    "cmd": "echo /dev/$(lsblk -no pkname \"/dev/sda1\")",
    "delta": "0:00:00.013634",
    "end": "2018-05-16 12:41:09.068166",
    "item": "/dev/sda1",
    "rc": 0,
    "start": "2018-05-16 12:41:09.054532"
}

STDOUT:

/dev/sda sda1
```

For instance, it will result with a devices list like:
`['/dev/sda sda1', '/dev/sdb', '/dev/sdc sdc1']`
where we expect to have:
`['/dev/sda', '/dev/sdb', '/dev/sdc']`

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1492242
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>
(cherry picked from commit 9cad113e2f22132d08208cd58462f11056c41305)