Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Standardized TRex client library path

    - path should be defined via TREX_CLIENT_LIB environmental variable e.g. TREX_CLIENT_LIB=/opt/trex_client/stl
    - refactored unit tests

    Change-Id: I18767e48daf774432c010f1b88d18a4f0ee4e156
    Signed-off-by: Maciej Skrocki <maciej.skrocki@intel.com>
    Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 2a51898..42e59b1 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 2a5189826106cee5bbbcc35df53abc4002c9d47f
+Subproject commit 42e59b19264a8a31650d2a111b79d932accd8e4e