Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "This is the vote for yardstick new PTL"
  - This is the vote for yardstick new PTL

    Ross Brattain who is yardstick commtter has been nominated as
    new PTL candidate before the nomination end date. As we only
    have one candidate, we will use gerrit to vote instead of civs

    all yardstick committers, please vote +2 or -2 before this friday
    the vote will be closed at that time

    Reference: https://wiki.opnfv.org/display/yardstick/Yardstick+PTL+Election

    Change-Id: I561f70051ed2593581ab909f08bd8be88c5b0e77
    Signed-off-by: kubi <jean.gaoliang@huawei.com>