Update git submodules

* Update docs/submodules/sdnvpn from branch 'master'
  - Add requirements in sdnvpn

    It now leverages on pbr to scan requirements.txt and inject them into
    the install_requires.

    Change-Id: Iae1517ae0bfd3575e9d16de4a2ca42e114d956fb
    Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>

diff --git a/docs/submodules/sdnvpn b/docs/submodules/sdnvpn
index de80ee5..4a6bb97 160000 (submodule)
--- a/docs/submodules/sdnvpn
+++ b/docs/submodules/sdnvpn
@@ -1 +1 @@
-Subproject commit de80ee56c458b127be3497f3882bc1c962f42dfe
+Subproject commit 4a6bb979b45c3b7f00f9dcc9519f439b3d405189