Update git submodules

* Update docs/submodules/releng from branch 'master'
  - Merge "jjb: xci: Refactor bifrost jobs to use the XCI clean VM"
  - jjb: xci: Refactor bifrost jobs to use the XCI clean VM

    We no longer run opnfv/bifrost jobs so drop all the references from
    the configuration file. Moreover, make use of the releng-xci XCI clean
    VM virtual machine to run the tests on a clean environment. Finally, we
    drop the support for uploading logs to the artifacts.opnfv.org. This
    needs to be done for XCI as well so we will bring it back when we
    develop a universal solution.

    Change-Id: I9bad17508ab8ee6594aec444ab864b99331b616d
    Signed-off-by: Markos Chandras <mchandras@suse.de>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index 19f559f..31af28e 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit 19f559f63eed668fcfa4d7ded964a635eb52a769
+Subproject commit 31af28e0d656645a6737c803d019d443d82e34d6