Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "IXIA traffic stops running after first iteration"
  - IXIA traffic stops running after first iteration

    IXIA traffic stops running after first iteration when
    traffic profile duration is set to 60 sec.

    JIRA: YARDSTICK-1374

    Change-Id: I216b98d43b66aabc72ed88e895b89e28572b264f
    Signed-off-by: Mytnyk, Volodymyr <volodymyrx.mytnyk@intel.com>
    Signed-off-by: Myron Sosyak <myronx.sosyak@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 771b041..87e8824 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 771b04149ff9417ba8784f0dab6aa3ab607a437b
+Subproject commit 87e8824086a5bc021b7708d36ad9968a8f1a29f0