Update git submodules

* Update docs/submodules/nfvbench from branch 'master'
  - Move Docker build to tag 1.3.1

    Change-Id: Ida8e9223300add358cfb5ed085274131da16934e
    Signed-off-by: ahothan <ahothan@cisco.com>

diff --git a/docs/submodules/nfvbench b/docs/submodules/nfvbench
index 0a0b2e9..e8a8983 160000 (submodule)
--- a/docs/submodules/nfvbench
+++ b/docs/submodules/nfvbench
@@ -1 +1 @@
-Subproject commit 0a0b2e9b3aaab1b3bd98b207ffde206eff2a513a
+Subproject commit e8a898337fcdc5e39721f187dabdfd0a51974986