Update git submodules

Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - cover.sh: delete .testrepository before running coverage

    When running py27 and py3 test ran into problems
    with .testrepository already exists, but testr
    thinking it was corrupt

    running testr
    No repository found in /home/jenkins/opnfv/slave_root/workspace/yardstick-verify-master. Create one by running "testr init".
    error: testr failed (3)

    The fix seems to be to delete .testrepository before
    running testr coverage

    Change-Id: Ib8cd3ab9d3384935380ac29ce365439c6464adc3
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>