Update git submodules

* Update docs/submodules/vswitchperf from branch 'master'
  - vpp: Define vppctl socket name

    Newer versions of VPP require an explicit definition of CLI socket
    file for proper functionality. Otherwise vppctl is not able to connect
    to running vpp process and thus vsperf is not able to configure
    and control vpp.

    Change-Id: Id0df75fd0894e22b1b4d7fa05e16f49efe20d3ab
    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>

diff --git a/docs/submodules/vswitchperf b/docs/submodules/vswitchperf
index 1653f5d..31770a6 160000 (submodule)
--- a/docs/submodules/vswitchperf
+++ b/docs/submodules/vswitchperf
@@ -1 +1 @@
-Subproject commit 1653f5d6a12febed50f138076dcf8c191d9f24e6
+Subproject commit 31770a64cd8a5c40ee3657ac97e87a900f7aeca5