purge: fix purge of lvm devices

using `shell` module seems to be the only way to make this task working
on rhel based distribution AND debian based distributions.

on ubuntu, using `command` ansible module fails like following
(not due to `sudo` usage or not):
```
ok: [osd1] => changed=false
  cmd: command -v ceph-volume
  failed_when_result: false
  msg: '[Errno 2] No such file or directory: ''command'': ''command'''
  rc: 2
```

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1653307

Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>

infrastructure-playbooks/purge-cluster.yml

@@ -304,7 +304,7 @@
     register: ceph_lockbox_partition_to_erase_path
 
   - name: see if ceph-volume is installed
-    command: "command -v ceph-volume"
+    shell: "command -v ceph-volume"
     failed_when: false
     register: ceph_volume_present