Update git submodules

* Update docs/submodules/nfvbench from branch 'master'
  - Change the signature for config validation function

    Change-Id: I4ef3d18f1b068165cdfc8d08278d12ec6eac4a4c
    Signed-off-by: Yichen Wang <yicwang@cisco.com>

diff --git a/docs/submodules/nfvbench b/docs/submodules/nfvbench
index 1fe5e7a..6c4e55c 160000 (submodule)
--- a/docs/submodules/nfvbench
+++ b/docs/submodules/nfvbench
@@ -1 +1 @@
-Subproject commit 1fe5e7adb3e1d90910c18386ed64d0a787a7bf94
+Subproject commit 6c4e55c53c65b7e63c17c85367b4813443c5d942