Update git submodules

* Update docs/submodules/yardstick from branch 'stable/danube'
  - Bugfix: yardstick env prepare cmd do not support other installer

    JIRA: YARDSTICK-629

    Currently yardstick env prepare do not support other installer.
    I will add follow support:
    1. Environment variable already exists: then do not fetch openrc file
    and do not add EXTERNAL_NETWORK variable.
    2. Already have openrc file in /etc/yardstick/openstack.creds: only
    source this file in API.
    3.Environment variable not exists and openrc file not in
    /etc/yardstick/openstack.creds: fetch openrc file and append
    EXTERNAL_NETWORK variable.

    Change-Id: I4ce98f2e17ef8e0a0a1c33c3862ca301c53bb6c2
    Signed-off-by: chenjiankun <chenjiankun1@huawei.com>
    (cherry picked from commit 48ad9b5dfa80c8d05d07c6f7fa12d53deddcf3a2)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index a87e2b8..6990400 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit a87e2b84d683d10ebace26f4df7a8bbe8a801e60
+Subproject commit 6990400d9c020ed0f600e02141efec883593c6c6