Update git submodules

Update git submodules

* Update docs/submodules/vswitchperf from branch 'stable/fraser'
  - trex: Add support for burst traffic type

    Support for burst traffic type was added into T-Rex. This
    traffic type is useful for tests, where a limited number
    of frames should be sent through DUT.

    JIRA: VSPERF-562

    Change-Id: I03b7150e66a0210cce91b20c751b8624c16f951b
    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: Richard Elias <richardx.elias@intel.com>
    (cherry picked from commit e9ff0fdf355d26d0c171c6c1137c4beb12561233)