Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "temp freeze releng version for stable/danube workaround"
  - temp freeze releng version for stable/danube workaround

    releng doesn't have a branch, so we need to freeze
    the releng version.

    There is an issue with releng fetch_os_creds.sh change

    /home/opnfv/repos/releng/utils/fetch_os_creds.sh: line 75: BRANCH:
    unbound variable

    So we need to revert back to commit before this change

    Change-Id: Ie8b0f68879a873aab584aa32b576d8b5ba602a13
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 8b8738e..418b310 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 8b8738ed675f8aab93d484df300e3d0c6c24becc
+Subproject commit 418b31051fb38881e3f18ad93d036878297e8ad7