Update git submodules

Update git submodules

* Update docs/submodules/releng-xci from branch 'master'
  - Merge "Bump bifrost sha and set UPPER_CONSTRAINTS_FILE"
  - Bump bifrost sha and set UPPER_CONSTRAINTS_FILE

    The lack of UC means that pip is attempting to install libvirt-python 4.1.0,
    which is failing on Ubuntu due to
        https://bugs.launchpad.net/openstack-requirements/+bug/1753539.
    Currently upper-constraints limits libvirt-python to 4.0.0, which does not
    have this issue.

    Change-Id: Idc97a0d9928cebafc9167614723236d2584cc93f
    Signed-off-by: Fatih Degirmenci <fdegir@gmail.com>