Update git submodules

* Update docs/submodules/yardstick from branch 'stable/danube'
  - Update cirros image to latest version

    Change-Id: Iffa1e4529e5b6e9664ac503a5815ea9a034d7515
    Signed-off-by: Michael Polenchuk <mpolenchuk@mirantis.com>
    (cherry picked from commit 043727feabaf7cabb719dbd3d52274bcd0dbea66)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 42f2d41..aa49906 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 42f2d416079ebe41a1f4d7a01cdb7896a871ad51
+Subproject commit aa499064bf305e41a56a2f86d17c1cc29a1d7b0c