Update git submodules

* Update docs/submodules/yardstick from branch 'stable/danube'
  - 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>
    (cherry picked from commit 6e77001bcd07f30f9edcf4103528002f4c4ec536)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index af00409..42f2d41 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit af004096fa17c9e29c663fc013228a9c82bd174d
+Subproject commit 42f2d416079ebe41a1f4d7a01cdb7896a871ad51