Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Added http concurrency test suite for agnostic VNF"
  - Added http concurrency test suite for agnostic VNF

    Added http concurrency test suite and test template.
    In the test pemplate user can set number of users and page size

    JIRA: YARDSTICK-1438

    Change-Id: I510c37309568c1ef9c4ab34d3b85614120dc79a8
    Signed-off-by: Taras Chornyi <tarasx.chornyi@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 0d06af4..78939bf 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 0d06af45d73df649afdabcd9a0b710295b0ea051
+Subproject commit 78939bfbd95b7047f00e234b03e478255ed6ece5