Update git submodules

Update git submodules

* Update docs/submodules/nfvbench from branch 'master'
  - Merge "Local Documentation Builds"
  - Local Documentation Builds

    This adds configuration for performing local documentation builds
    with the following simple command:

    tox -e docs

    This patch is part of our documentation effort to Move away from
    submodules
    https://docs.opnfv.org/en/latest/how-to-use-docs/local-build-transition.html

    Once merged docs will be viewable at opnfv-nfvbench.readthedocs.io

    Change-Id: I7c877847ec23c4cfc44ff7f106ed2acc65688a41
    Signed-off-by: Aric Gardner <agardner@linuxfoundation.org>