Update git submodules

* Update docs/submodules/vswitchperf from branch 'stable/euphrates'
  - trex: RFC2544 Throughput bugfixing

    Changes introduced by patch:
    * Ensure that correct results with given lossreate are returned
    * Add threshold value to specify the accurancy of RFC2544 Throughput
      measurement. Default value is 0.05.

    JIRA: VSPERF-530

    Change-Id: I8148deba9039c3a0feb6394d6671aa10c5afaf0a
    Signed-off-by: Martin Goldammer <martin.goldammer6@gmail.com>
    Reviewed-by: Martin Klozik <martinx.klozik@intel.com>
    Reviewed-by: Al Morton <acmorton@att.com>
    Reviewed-by: Christian Trautman <ctrautma@redhat.com>
    Reviewed-by: Trevor Cooper <trevor.cooper@intel.com>
    (cherry picked from commit f296fd2a44a1eb947e0b0ddecf017231d05abece)

diff --git a/docs/submodules/vswitchperf b/docs/submodules/vswitchperf
index f2cf028..a91c1c1 160000 (submodule)
--- a/docs/submodules/vswitchperf
+++ b/docs/submodules/vswitchperf
@@ -1 +1 @@
-Subproject commit f2cf028bcb55b868f62fd25c692bd32dd92c27ef
+Subproject commit a91c1c1a1c31cd049f3cfeb665ea6d0b7aeec7de