Update git submodules

Update git submodules

* Update docs/submodules/fuel from branch 'master'
  - [build] Use python -m pip workaround

    pipenv depends on pip, which conflicts with some distro versions of
    pip, see bug [1].

    While at it, workaround lib64 issues on CentOS by passing
    `--always-copy` when creating the venv [2].

    [1] https://github.com/pyenv/pyenv/issues/1141
    [2] https://github.com/pypa/pipenv/issues/1929

    Change-Id: I6af1a5fdbe8d37f75ab8b09b050449ad432191ad
    Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>