Update git submodules

Update git submodules

* Update docs/submodules/releng-xci from branch 'master'
  - Merge "bifrost: Optimize the OPNFV bifrost playbook"
  - bifrost: Optimize the OPNFV bifrost playbook

    We could combine several plays into a single one when they are executed
    to the same set of nodes. This has the added benefit that we avoid all
    the Ansible warm-up operations when a new play is started.

    Change-Id: Ibcad07d1b74f1d7cc5a9c60e4b2a6130fb6178fd
    Signed-off-by: Markos Chandras <mchandras@suse.de>