Update git submodules

Update git submodules

* Update docs/submodules/releng-xci from branch 'master'
  - bifrost: Use the pre-built XCI OS images instead of dib

    We already have images for all 3 distros which we could simply re-use
    for all the XCI VM deployments instead of building new ones everytime
    with DIB. The images will be copied to the new VM from the cache
    directory if they are available otherwise we will simply download them
    during the XCI execution phase.

    Change-Id: I2a8391650558511668654c6b54a10db316f867a2
    Signed-off-by: Markos Chandras <mchandras@suse.de>