Update git submodules

* Update docs/submodules/yardstick from branch 'stable/fraser'
  - Merge "Bugfix: Can't get image list in API" into stable/fraser
  - Bugfix: Can't get image list in API

    JIRA: YARDSTICK-1110

    To match OpenStack Pike release, we upgrade novaclient from 7.1.1 to 9.1.1.
    And the client interface has changed, it no longer has nova_client.images attribute.
    So I use shade instead. Later will change all same issue.

    Change-Id: Ie4f54069d4346e44e2ad925439930504b945cbad
    Signed-off-by: chenjiankun <chenjiankun1@huawei.com>
    (cherry picked from commit 5ac59e495d74b64ab377f1ba25bb00b11da14e67)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 2fe0cfa..73b611a 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 2fe0cfa88f1792acb11fd875f7dd2fc9940a3866
+Subproject commit 73b611a7683f1812ab8af715662d0b155d6e1e70