Update git submodules

Update git submodules

* Update docs/submodules/releng-xci from branch 'master'
  - Change variable in start-new-vm.sh

    Since the patch:

    https://gerrit.opnfv.org/gerrit/#/c/63173/

    The variable BIFROST_USE_PREBUILT_IMAGES has changed to
    BIFROST_CREATE_IMAGE_VIA_DIB. As jenkins does not trigger testing jobs
    when editing file in xci/scripts/, this change is done in a separate
    patch

    Change-Id: I3bc285936fae5b7514272ca0ad2418b60446e4aa
    Signed-off-by: Manuel Buil <mbuil@suse.com>