Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Set "cmd2" library to version 0.8.6"
  - Set "cmd2" library to version 0.8.6

    Last "cmd2" version, 0.9.0, removed Python 2+ support [1]. In order to
    support Python 2.7, version 0.8.6 (or earlier) must be installed.

    [1] https://github.com/python-cmd2/cmd2/commit/e54e5a5b891548df4df0acb04b6e607ad8f871b1

    JIRA: YARDSTICK-1205

    Change-Id: Ic2fe57438720de5b21c75f9456e653c6facf3ae9
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>