Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "test_nstat: speedup unittests, sleep for 0 seconds"
  - test_nstat: speedup unittests, sleep for 0 seconds

    Change-Id: I3f961e26fb045ea7501dc3a5b630b3ef94fec3ee
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 48c0cb1..ad60cea 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 48c0cb1b5f3ccd023df1c4ae708236d2f459bfa8
+Subproject commit ad60cea13e6b8a280636c289ef8a79f7e0f40de0