Update git submodules

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>