Update git submodules

* Update docs/submodules/releng from branch 'master'
  to d8cf647af9bc249587e86680bbef26192e5adaaa
  - Install python3-venv for nfvbench

    Change-Id: I719c4c590992777755c038c1b393851a5dbc8464
    Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index 9aabfee..d8cf647 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit 9aabfee9ad45ba409721a1a609d58747efc4fcd9
+Subproject commit d8cf647af9bc249587e86680bbef26192e5adaaa