Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Remove generated file in 'test_build_vm_xml'"
  - Remove generated file in 'test_build_vm_xml'

    Change I22e95c488e27d6e2a8fdf6c1a07faab275fa6bba introduced a change in
    test case 'test_build_vm_xml' [1]. This test case generates a file which
    is stored locally but never removed.

    [1]https://github.com/opnfv/yardstick/blob/80dc9034edeb05f66bfa6264ae0adf0f6aea60da/tests/unit/benchmark/contexts/standalone/test_model.py

    JIRA: YARDSTICK-918

    Change-Id: I91d39bb37c3d8a273fe68155bb308c5a3bc1ca9d
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index e67082e..868db57 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit e67082ecf9c89eeebbdf2c87b0a90e3edef2eabd
+Subproject commit 868db57177cdbbebf8d160ca049cdf81fda3a2f4