Update git submodules

* Update docs/submodules/releng from branch 'master'
  - Revert "Run vnfs in parallel in gates"

    Our AIO doesn't have enough resources to run all VNFs in parallel [1].

    [1] https://build.opnfv.org/ci/job/functest-opnfv-functest-vnf-latest-heat_ims-run/98/console

    This reverts commit 2fed59b78a0ccf62364a6af8722c194a0160375f.

    Change-Id: Ibf7cd31460bb8d1fb3e560694829a913e1cec1c0
    Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index 25a313f..72dc6fe 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit 25a313f538e5fb1fcd8a3203c75d744ef07f074d
+Subproject commit 72dc6fee623fdf32fa8f5ebbcae02f6cb9a4b088