Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Remove "flake8" and "pep8" from test-requirements"
  - Remove "flake8" and "pep8" from test-requirements

    Removed "flake8" and "pep8" from test-requirements. Since
    44070670c798d2cde2824f3150b2b11bf35c6319, "pylint" is the
    code style checker, instead of "flake8" or "pep8".

    JIRA: YARDSTICK-832

    Change-Id: Iee598b0b1793d71dc74be36fa1875387b17d1e25
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>