Update git submodules

* Update docs/submodules/yardstick from branch 'stable/danube'
  - Update virtualenv installation step in userguide

    Change-Id: I0b3e8f22b6878c3b2cf1a54f3c39f2888406c508
    Signed-off-by: JingLu5 <lvjing5@huawei.com>
    (cherry picked from commit f6e13de54c5b71f9112c38b12f6c327af743d68a)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 7d08b88..48d8c77 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 7d08b884344651d9ba3dd2830dcd976d2adba9f5
+Subproject commit 48d8c77c4989301847c70fba6fa589fb0316e05f