Update git submodules

* Update docs/submodules/releng from branch 'master'
  to a78697d4c2304ee4dcc56f3d8580a02d27dc00e1
  - Override the right rally blacklists

    Latest containers now leverage on python3.6.

    Change-Id: Ib3b3c02497b85dd210713baed345c4a64a0add5b
    Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index 7a5fddd..a78697d 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit 7a5fddd061b4573d7e96ad7d6c49f456bb17e988
+Subproject commit a78697d4c2304ee4dcc56f3d8580a02d27dc00e1