Update git submodules

Update git submodules

* Update docs/submodules/sdnvpn from branch 'master'
  - Merge "Put the ubuntu images in new directory"
  - Put the ubuntu images in new directory

    To adopt the new change of Functest[1], all images are put under
    ${HOME}/functest/images. Users are allowed to
    pre-download the images and map the local folder to the new directory.

    [1] https://gerrit.opnfv.org/gerrit/#/c/34033

    Change-Id: Iafee8825af77dfce74ad2ace3346e3f2b70a7c96
    Signed-off-by: helenyao <yaohelan@huawei.com>