Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Ensure that at least one handler is available"
  - Ensure that at least one handler is available

    https://jira.opnfv.org/browse/YARDSTICK-773?filter=-3

    Remove dependency of yardstick on utils methods

    Change-Id: Iadf502364a7f08c279a8f0d17d7e45e8047f4066
    Signed-off-by: Edward MacGillivray <edward.s.macgillivray@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index e8cf6a7..dd57115 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit e8cf6a76c346806b53fa0c802374327ddc4956d1
+Subproject commit dd57115618caa299f6dd1eb350b55ecf062ca92e