Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Add a new monitor type: MultiMonitor that can run any number of other monitors at the same time."
  - Add a new monitor type: MultiMonitor that can run any number of other monitors at the same time.

    JIRA: YARDSTICK-397

    Change-Id: Ic5cb79f0820029e306373abead1ea43fac9abee2
    Signed-off-by: HuanLi <lihuansse@tongji.edu.cn>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index d002db9..87559e9 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit d002db9a10fed7cdba2cc43a8c064864250994ce
+Subproject commit 87559e992c1f68559950b258146a530c49db9df0