Update git submodules

* Update docs/submodules/sdnvpn from branch 'stable/danube'
  - Changing to new docs structure

    Change-Id: I4c15e1635981b2b5fe5473be89b5abab78bfabd5
    Signed-off-by: Nikolas Hermanns <nikolas.hermanns@ericsson.com>
    (cherry picked from commit 0f8220d5d96cc0a45be00e937c73ab938133e587)

diff --git a/docs/submodules/sdnvpn b/docs/submodules/sdnvpn
index 9101d99..2bec15d 160000 (submodule)
--- a/docs/submodules/sdnvpn
+++ b/docs/submodules/sdnvpn
@@ -1 +1 @@
-Subproject commit 9101d99bbf08e40a76651ca0e56c01ba3581f344
+Subproject commit 2bec15d7d92172681771fd4567b959662087d7b7