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>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index 8bf1237..8d56e29 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit 8bf12376cc1ba62a4d26967d074a6de12996d644
+Subproject commit 8d56e29f3dd1fad74a678202a70eb9d8bbf00bfc