Update git submodules

Update git submodules

* Update docs/submodules/releng-xci from branch 'master'
  - xci: bifrost: Fix unbound variable error

    If we are not in a virtual environment, then VIRTUAL_ENV is going
    to be unset and we will fail with the following error:

    ./scripts/bifrost-provision.sh: line 105: VIRTUAL_ENV: unbound variable

    As such, we need to provide a default value if that variable is unset.
    We also fix a typo in the _sudo variable.

    Change-Id: Ic018716c03ffba6e8e742e34720ea462b85341be
    Signed-off-by: Markos Chandras <mchandras@suse.de>