library/ceph_volume.py: remove six dependency

library/ceph_volume.py: remove six dependency

The ceph nodes couldn't have the python six library installed which
could lead to error during the ceph_volume custom module execution.

  ImportError: No module named six

The six library isn't useful in this module if we're sure that all
action variables passed to the build_ceph_volume_cmd function are a
list and not a string.

Resolves: #4071

Signed-off-by: Dimitri Savineau <dsavinea@redhat.com>