Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Remove hw_video_model and hw_firmware_type from metadata"
  - Remove hw_video_model and hw_firmware_type from metadata

    Starting with the Openstack Queens release,
    hw_firmware_type=uefi is implied on AArch64 by default
    Using virtio-video, now default in nova, should be used
    instead of VGA

    JIRA: ARMBAND-388

    Change-Id: If52206e2a2f0a05bde5632c91de9367fffdc2eb3
    Signed-off-by: Delia Popescu <delia.popescu@enea.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index af0a6a7..81c791a 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit af0a6a7593534849626000632b2262cf5fe76db7
+Subproject commit 81c791a5598baba57d829f338abde29f7b157b71