Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Upgrade collectd to version 5.8

    Update collectd version to 5.8 as it's the latest stable
    and supports GCC 7

    JIRA: YARDSTICK-1114

    Change-Id: I9147b7277d87047eda729664d5ee3e8ab2c145af
    Signed-off-by: John O Loughlin <john.oloughlin@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index d2e5307..64c392a 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit d2e5307fed153989558aa9506f33e69c74d29675
+Subproject commit 64c392afd017b412e8df9eda8ab08093b9933e6e