Update git submodules

* Update docs/submodules/releng from branch 'master'
  - Merge "prototypes: bifrost: Switch Ubuntu targets to Xenial (16.04)"
  - prototypes: bifrost: Switch Ubuntu targets to Xenial (16.04)

    Switch default target OS to Ubuntu Xenial (16.04) which is also
    the version which is supported by OpenStack Ansible.

    Change-Id: I73adc6472b0fb572177e1508cf831b4d3190245c
    Signed-off-by: Markos Chandras <mchandras@suse.de>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index 5b9d8e0..6a2b8d4 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit 5b9d8e03f86a1e5c9f8e36c3e8945c324848976e
+Subproject commit 6a2b8d4f1ddd0a2b1a11c9b51518e6ea56330774