ceph-volume: remove --root param from nsenter cmd

ceph-volume: remove --root param from nsenter cmd

This is redundant and makes nsenter throw messages like following:
```
  Failed to find sysfs mount point
  dev/block/11:0/holders/: opendir failed: Not a directory
  dev/block/252:0/holders/: opendir failed: Not a directory
  dev/block/253:0/holders/: opendir failed: Not a directory
  dev/block/252:1/holders/: opendir failed: Not a directory
  dev/block/253:1/holders/: opendir failed: Not a directory
  dev/block/252:2/holders/: opendir failed: Not a directory
  dev/block/253:2/holders/: opendir failed: Not a directory
  dev/block/252:3/holders/: opendir failed: Not a directory
  dev/block/253:3/holders/: opendir failed: Not a directory
  dev/block/252:16/holders/: opendir failed: Not a directory
  dev/block/252:32/holders/: opendir failed: Not a directory
  dev/block/252:48/holders/: opendir failed: Not a directory
  dev/block/252:64/holders/: opendir failed: Not a directory
  ```

Fixes: https://tracker.ceph.com/issues/52926
Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>