Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Bugfix: improper license header in swagger docs

    JIRA: YARDSTICK-609

    In the patch:
    https://gerrit.opnfv.org/gerrit/#/c/28833/

    It update the license header, but it is improper to swagger yaml file.
    The license information will show in swagger docs.
    So I remove it.

    Change-Id: Ib87f0f43a95e3b2145c4b59059c4573000f1997c
    Signed-off-by: chenjiankun <chenjiankun1@huawei.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 2eca9b5..6e77001 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 2eca9b5a2de0e2c02e7ba3991944f01761679d3f
+Subproject commit 6e77001bcd07f30f9edcf4103528002f4c4ec536