Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Trex KPI are not collected due to rabbitmq user"
  - Trex KPI are not collected due to rabbitmq user

    Changed the way rabbitmq-server user is created. Fix is needed to
    collect KPIs from Trex.

    JIRA: YARDSTICK-1479

    Change-Id: I54369535fdb7af6af76de30f758227736f83db78
    Signed-off-by: Stepan Andrushko <stepanx.andrushko@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 56b4d5d..87c09c6 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 56b4d5de28bd8f491cea077e21811830959b6ea8
+Subproject commit 87c09c6477e7e351a821d27013031fafa5c37615