Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "requirements: bump Ansible version to 2.3"
  - requirements: bump Ansible version to 2.3

    Change-Id: Ib429ba24d2b7287b6ec4e749386da0e1242d6a20
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 36a2e04..870d2ba 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 36a2e04b468d980c98bd43768e4c88d37a8a69f1
+Subproject commit 870d2ba61b4a3352ef8485c3b39690346010e89b