Update git submodules

* Update docs/submodules/ccsdk/platform/plugins.git from branch 'master'
  to 6a407a45c77662a42160462c8fa69261f3fc76b3
  - Merge "Support python3 in all plugins"
  - Support python3 in all plugins

    Unify tox/requirements/setup.py requirement specifications.
    Use cloudify-common 5.0.5 release.
    For helm, use a dev cloudify-common build with better python3 code.
    Use PEP 508 URLs in requirements for the non-PyPI (github) release.
    Fix mixed indentation and trailing whitespace.

    Signed-off-by: Miroslav Los <miroslav.los@pantheon.tech>
    Issue-ID: CCSDK-1931
    Change-Id: I607957d9e2c299121785f3f02420c6038966a200