Update git submodules

Update git submodules

* Update docs/submodules/sdnvpn from branch 'stable/danube'
  - Update url for ubuntu image downloading- testcase3

    The url that the testcase3 uses to download ubuntu image
    (https://cloud-images.ubuntu.com/releases/16.04/release/
    ubuntu-16.04-server-cloudimg-amd64-disk1.img) is unstable.

    This change updates it, using the one from the OPNFV artifact repo

    JIRA: SDNVPN-133

    Change-Id: Ic7265b572064f1df7ce30ab5249b1bdb61c63777
    Signed-off-by: tomsou <soth@intracom-telecom.com>
    (cherry picked from commit 70e78577e8736d17fe3d99c9c276d7ca9b6ea89b)