Update git submodules

Update git submodules

* Update docs/submodules/sdnvpn from branch 'master'
  - [Bug Fix] Use correct image for the Quagga instance

    In testcase_3 the Quagga image is downloaded from the
    opnfv.artifacts repository to the /home/opnfv/functest/data
    directory.

    But the testcase_3 after that tries to find the image in the
    /home/opnfv/functest/images directory.

    This results in using a wrong image

    Jira: SDNVPN-213

    Change-Id: I99539fc9a7a2d016a97ed51fca0cde81f82b3911
    Signed-off-by: Dimitrios Markou <mardim@intracom-telecom.com>