Update git submodules

* Update docs/submodules/yardstick from branch 'master'
  - Add sample OpenStack/SRIOV RFC2544 based TC

    Add sample OpenStack/SRIOV RFC2544 based test case which can be
    run using external Trex.

    Console log: http://paste.openstack.org/show/658002/
    Result output: http://paste.openstack.org/show/658004/

    JIRA: YARDSTICK-967

    Change-Id: Id0e2d6d47f459c7e6a3370735bab74f7ce502b24
    Signed-off-by: Mytnyk, Volodymyr <volodymyrx.mytnyk@intel.com>

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index fa6a71d..e91789d 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit fa6a71d6a0f654d9e85a6dd8f73c1173f75ccdde
+Subproject commit e91789dc2832a685c58591a9890ddbecdf52ae2d