Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "validate_non_string_sequence: use six.string_types"
  - validate_non_string_sequence: use six.string_types

    Change-Id: I7248d97a55974091daf20f0fba4b612101a0e4c5
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index b90dd79..f6bc49d 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit b90dd796629bcc82cea5e10eb910f70deb3e2bf0
+Subproject commit f6bc49db9b164b8e8c9bba99b1d513edc2cc610f