Update git submodules

* Update docs/submodules/vswitchperf from branch 'master'
  - install: Python virtualenv python3 enforcement

    It was reported that virtualenv-3.x sometimes uses python2.7 instead of
    python3. This update of the python virtual environment creation scripts,
    enforces the use of python3 from /usr/bin/python3.
    The scripts were tested on fresh installations of each currently
    supported Linux OS.

    JIRA:VSPERF-551

    Change-Id: If909dd9578a3c42b14c1a3e4f9ed7b07a675fb58
    Signed-off-by: Richard Elias <richardx.elias@intel.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: Sridhar Rao <sridhar.rao@spirent.com>
    Reviewed-by: Trevor Cooper <trevor.cooper@intel.com>

diff --git a/docs/submodules/vswitchperf b/docs/submodules/vswitchperf
index caabba9..21b2352 160000 (submodule)
--- a/docs/submodules/vswitchperf
+++ b/docs/submodules/vswitchperf
@@ -1 +1 @@
-Subproject commit caabba96ca0efb5e0d21ce5122da4e5fc873f4fe
+Subproject commit 21b23522861e364daf150cd997c5d60851847ef2