Update git submodules

* Update devstack-plugin-ceph from branch 'master'
  to c6de66193950bbdf6734700aabb7d54ad5e59e01
  - Changed minversion in tox to 3.18.0
    
    The patch bumps min version of tox to 3.18.0 python in order to
    replace tox's whitelist_externals by allowlist_externals option:
    https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23
    
    Change-Id: I4bc8f70dd208e39c41d68bb82cf54ccaea9f6ebf