Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "update docker version to 16.04"
  - update docker version to 16.04

    JIRA: YARDSTICK-690

    This patch is for Yardstick Docker base image upgrade to 16.04

    This patch will enable ansible build image in yardstick env prepare.

    Add qemu-img convert to ansible build image.

    Change-Id: I43127b6020bd20bd9f4aac4fca0df75353b24346
    Signed-off-by: Ace Lee <liyin11@huawei.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 31b02e5..b77826d 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 31b02e5b95d8b4076ef60667d051a8a1a6b072ba
+Subproject commit b77826de37bde8683f15bd758951dfaf8af6402d