Update git submodules

Update git submodules

* Update docs/submodules/functest from branch 'master'
  - Merge "Give a list of links to wget"
  - Give a list of links to wget

    Now download_images.sh simply relies on wget -i.
    It downloads all requirements as the biggest ones are mandatory or
    scenario independent (VNF)

    The offical ubuntu-16.04-server-cloudimg-amd64-disk1.img is
    selected instead of the one hosting by OPNFV SDNVPN.

    Change-Id: Ica566bb351f22dfba001678fe93b8a2b1523941a
    Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>