Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Bugfix: yardstick env prepare cmd do not support other installer"
  - 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>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 90032ba..6c2f045 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 90032ba61345752b014002dda629f84c42780e6c
+Subproject commit 6c2f0450f159ce67bc0b2c5c2bfe83aab16b5750