Update git submodules

* Update docs/submodules/yardstick from branch 'stable/fraser'
  - Merge "Error when adding SR-IOV interfaces in SR-IOV context" into stable/fraser
  - Error when adding SR-IOV interfaces in SR-IOV context

    Method "add_sriov_interfaces" in Libvirt class throws an error when trying
    to add SR-IOV interfaces in the XML file.

    Function "setup_sriov_context" in SriovContext if passing to
    "_enable_interfaces" method the XML file name instead of passing the XML
    string.

    JIRA: YARDSTICK-1189

    Change-Id: If021c102311ecb98cb26934f78067dfc720e4291
    Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>
    (cherry picked from commit 72df0eaac4a2a1f23439af7f249c2b5d18d1b819)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index 0af2b92..fb641de 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit 0af2b92e25345ec15649424d786142ea126e66ac
+Subproject commit fb641dea618c5b2530abf1e920f3f599de9eb59b