Update git submodules

* Update docs/submodules/releng from branch 'master'
  to 3aad5c124e77b44b513727281e471d7083eb190b
  - Select lf-virtual1 as defaut slave (ut)

    It allows verifying unit tests vs python3 and building docs (see
    latest OpenStack upper-constraints)

    Change-Id: Ie9566d4b1e0a288d5944c3111ec2dc0749be1975
    Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index 09c9054..3aad5c1 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit 09c905482f1c4578a94c420057bc391718d508c2
+Subproject commit 3aad5c124e77b44b513727281e471d7083eb190b