Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Bugfix: remove ping package for it is GPL license

    JIRA: YARDSTICK-777

    In migrate test case, we use ping to count downtime.
    But ping package is GPL license, so we need remove it.
    Since ping command tool can not set timeout less then 1 second, and the
    downtime is less then 1 second, so we need find another package to replace.
    For now, I find most ping tool is GPL license, so maybe later we need to
    implement ping ourselves.
    And the migrate case can not calculate downtime unless user install ping
    themselves.

    Change-Id: I78cba5e4af9de2e3c0bb16b27570bed951caf440
    Signed-off-by: chenjiankun <chenjiankun1@huawei.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 5ce3b6f..a862619 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 5ce3b6f8c8b3217091e51a6041455738603d90b8
+Subproject commit a8626195f5e8530f989693a7d8dc73f97ebe115b