Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Merge "Replace subprocess "check_output" with "Popen""
  - Replace subprocess "check_output" with "Popen"

    "check_output" is a blocking wrapper for "Popen" which returns the output
    of the command execution or raises an exception in case of error.

    "Popen" is a non-blocking function that allows to create asynchronous
    tasks. It returns any possible execution error but doesn't raise an
    exception; this is delegated to the developer.

    This code is used in the Yardstick CLI base test class.

    Change-Id: Ie3e1228b2d40cb306354447653678bf581bc9697
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index edb1b87..3984c97 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit edb1b87fae318501b853ef02a7aa7dffddfe1e48
+Subproject commit 3984c976967e3d697473cd3bb85204577a838dcd