Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Assign IXIA ports in one call instead of one by one

    Assigning IXIA ports one by one takes more time than
    assign all ports by once. Also, assigning ports in
    one call resolves the assign port issues caused on
    some IXIA chassis versions.

    JIRA: YARDSTICK-1425

    Change-Id: Ibb08e40ee1a8762573f6752bdd1f583c5a1d9d2a
    Signed-off-by: Mytnyk, Volodymyr <volodymyrx.mytnyk@intel.com>
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 7dfd3b0..ca7f539 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 7dfd3b0f70dc6ee7011031aa43606d679831db90
+Subproject commit ca7f539f1071d925676273c14daf6137685a3d91