Update git submodules

* Update docs/submodules/yardstick from branch 'stable/danube'
  - Merge "Add a new monitor type: MultiMonitor that can run any number of other monitors at the same time." into stable/danube
  - 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: LiHuan <lihuanwk@126.com>
    (cherry picked from commit 747a3260fef52fb5f7da337bc149ec202a05be6e)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 632dc80..e1b315f 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 632dc80c3ca42fe7f3d041c645eabef284f147e8
+Subproject commit e1b315fb5fded93c03693ac097b1a749dcc50034