Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Collectd Fixes"
  - Collectd Fixes

    This fixes collectd installation as well as configuration:
    1) Proxy fix, avoid overriding env proxy if already specified under
       'http_proxy' env var and not specified on cmd
    2) Install 'libvirt-dev' for 'virt' collectd plugin
    3) Using dpdk already downloaded by nsb_setup rather than getting latest
       , because master branch dpdk is failing while building eal_memory
    4) Fix typo while checkout new branch

    Change-Id: I0cc36f49d48d80f7edc35634dfe77016bd35978a
    Signed-off-by: Dhaval Patel <dhaval.r.patel@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 57b17a2..a66b78d 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 57b17a2223d8751f2c5eae660b706e2fc736a47c
+Subproject commit a66b78d60d7d062218df8e6b9c279f7931a030cd