Update git submodules

* Update docs/submodules/yardstick from branch 'stable/fraser'
  - Merge "Remove instantiated contexts in "test_task"" into stable/fraser
  - Remove instantiated contexts in "test_task"

    During unit testing, all contexts instantiated must be deleted at the end
    of the test to avoid clashes with other tests.

    JIRA: YARDSTICK-1209

    Change-Id: I1994b915001b4a518e096833a0c5d33febbd8de6
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>
    (cherry picked from commit 5e4d95ba299ce92588701d9481657c343b8ef609)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index d2c984e..53e84f5 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit d2c984e806eac4c981eec95626884c48eb699299
+Subproject commit 53e84f535ca03e7aabb902e8f21a77171fe40c1b