Update git submodules

Update git submodules

* Update docs/submodules/releng from branch 'master'
  - Merge "prototypes: bifrost: Remove /opt/stack from previous deployments"
  - prototypes: bifrost: Remove /opt/stack from previous deployments

    /opt/stack is being created by bifrost so the destroy-env.sh
    script should be responsible for cleaning up all the remnants.
    That way we can use the destroy-env.sh script outside of Jenkins
    as well.

    Change-Id: I72d9737cf2446ff3ffc02f195337a6873eceb5c0
    Signed-off-by: Markos Chandras <mchandras@suse.de>