Update git submodules

* Update docs/submodules/sdnvpn from branch 'stable/fraser'
  - Fix os-odl-bgpvpn-[no]ha scenario reference

    These shouldn't be prefixed with a '-' nor duplicated.

    Change-Id: I88f5b2d18436a258d6ac573a1fbc6dee735b48da
    Signed-off-by: Trevor Bramwell <tbramwell@linuxfoundation.org>

diff --git a/docs/submodules/sdnvpn b/docs/submodules/sdnvpn
index 23ec591..db5541b 160000 (submodule)
--- a/docs/submodules/sdnvpn
+++ b/docs/submodules/sdnvpn
@@ -1 +1 @@
-Subproject commit 23ec5914be722401dcbc2ceb9f4be1491c150776
+Subproject commit db5541bb9fc1000ebec787b356bf7d6d26165bf6