Update git submodules

Update git submodules

* Update docs/submodules/vswitchperf from branch 'master'
  - ixia: Fix IXIA in trafficgen mode

    There are two IXIA wrappers in VSPERF - IxNET and Ixia. It has
    been found that legacy Ixia implementation is no longer working
    in trafficgen mode. This issues has been fixed.

    JIRA: VSPERF-513

    Change-Id: Ief1db71f688a19c5b41508617ccbc73024a02c48
    Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
    Reviewed-by: Al Morton <acmorton@att.com>
    Reviewed-by: Christian Trautman <ctrautma@redhat.com>
    Reviewed-by: Sridhar Rao <sridhar.rao@spirent.com>
    Reviewed-by: Trevor Cooper <trevor.cooper@intel.com>