Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Error in string format in HeatTemplateError message"
  - Error in string format in HeatTemplateError message

    There is a missing "s" in HeatTemplateError message [1].

    [1]https://github.com/opnfv/yardstick/blob/8400c1895c49cb0c7802692f75d820fb5b8b7dc4/yardstick/common/exceptions.py#L70

    JIRA: YARDSTICK-1047

    Change-Id: Ieb6fbf0af35d624015dbf34f34de7c7cc9145dd5
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index d9a19da..7a78031 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit d9a19da691a0cc258d26a2967983396b1c034b65
+Subproject commit 7a78031d2a8daabd880e33b26bc7414780f36a14