Update git submodules

* Update docs/submodules/releng from branch 'master'
  - Merge "Change the path of images from pre_config/ to images/"
  - Change the path of images from pre_config/ to images/

    1. change the path of all images
    2. remove the sdnvpn special envs

    This patch should be merged after these 2 patches.
    1. https://gerrit.opnfv.org/gerrit/#/c/53395/
    2. https://gerrit.opnfv.org/gerrit/#/c/53757/

    Change-Id: Ie1dc9388be52629c5fcc0ef893963c4de2c72491
    Signed-off-by: xudan <xudan16@huawei.com>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index 5af2914..abd754f 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit 5af2914b837c9559d04c83d7db3f9eb061e47118
+Subproject commit abd754f9358035062d977c46b4e9e0a171cf5261