Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "[docs] Test case documentation for vEPC testing"
  - [docs] Test case documentation for vEPC testing

    JIRA: YARDSTICK-1349

    The test case document will cover the following:
    - test purpose
    - test configuration
    - pre-test conditions
    - test sequence
    - test verdict

    Change-Id: I2841a7ff3d248bb3e4739579fad722dc02b5504f
    Signed-off-by: Abhijit Sinha <abhijit.sinha@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 61942da..bdf55c2 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 61942da1ebc55893a139d34ae03244a2e776fcbd
+Subproject commit bdf55c2a6f000e0ca6462a9df4a0d411c3dd029e