Update git submodules

* Update docs/submodules/sdnvpn from branch 'master'
  - Merge "Fix reference-before-assignment errors when getting exceptions"
  - Fix reference-before-assignment errors when getting exceptions

    JIRA: SDNVPN-228

    This patch checks that variables have been set before trying to
    perfom actions uppon them.

    Change-Id: I6e81a3e7756d3ef0aee4f436766128631439f43a
    Signed-off-by: Stamatis Katsaounis <mokats@intracom-telecom.com>

diff --git a/docs/submodules/sdnvpn b/docs/submodules/sdnvpn
index 223484a..6ffe81b 160000 (submodule)
--- a/docs/submodules/sdnvpn
+++ b/docs/submodules/sdnvpn
@@ -1 +1 @@
-Subproject commit 223484a44ea3d59c7e8a5e788fecc7f0545b0331
+Subproject commit 6ffe81bf06727cb49be31cc0dd87aa506a48a834