Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Clean-up tests in test_vsperf_dpdk.py:VsperfDPDKTestCase"
  - Clean-up tests in test_vsperf_dpdk.py:VsperfDPDKTestCase

    * Move some common mocking into the setUp()
      The same return values was being set for moch_ssh
      in each test method.
    * Remove duplicate statements in test_vsperf_dpdk.py:VsperfDPDKTestCase
      The same assert* were being called for multiple methods,
      which were checking that setup worked as expected.
      These checks are not required each time.

    Change-Id: I325fc98ee569dcf7988951c82da0c02d5e465a16
    Signed-off-by: Emma Foley <emma.l.foley@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 61d471c..a2539c8 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 61d471c367ff61cbac080bd1708decf1b36ad167
+Subproject commit a2539c884c5f21925cf0533d5c9e65c8cc3488cf