Update git submodules

* Update docs/submodules/yardstick from branch 'stable/euphrates'
  - bugfix: kill process do not accurately kill "nova-api"

    JIRA: YARDSTICK-849

    1. update the kill process RegEx pattern to be more accurate
    2. make attack_process to be parameter so it can be customized if
    the SUT is using a different name of attack_process

    Change-Id: I569730ced6c24aafbffcf2fc5752d3560d0adac5
    Signed-off-by: rexlee8776 <limingjiang@huawei.com>
    (cherry picked from commit 4bbd919e2b26fe606bb1b83efc579e0f2557593e)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 2cbda68..7bad248 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 2cbda68cf08d9319cf88a1b221b9c7e55a09d124
+Subproject commit 7bad2481c956956ce62eff953ae477333b6c4494