Update git submodules

* Update docs/submodules/sdnvpn from branch 'master'
  - create log file only if it doesn't exist

    Change-Id: I97e4f0cf8f27ff719253967ac14e74bcd55b0559
    Signed-off-by: Periyasamy Palanisamy <periyasamy.palanisamy@ericsson.com>

diff --git a/docs/submodules/sdnvpn b/docs/submodules/sdnvpn
index c1e5aaf..9df57f9 160000 (submodule)
--- a/docs/submodules/sdnvpn
+++ b/docs/submodules/sdnvpn
@@ -1 +1 @@
-Subproject commit c1e5aafd5b5984b995d3ac6c74703e6a8be6e38b
+Subproject commit 9df57f96b44d41c98a556a790dfa7f96b8dc9808