Update git submodules

* Update docs/submodules/releng from branch 'master'
  - Revert "[fuel] Define Docker tag for stable branches"

    Moved logic to OPNFV Fuel via [1].

    [1] https://gerrit.opnfv.org/gerrit/#/c/65833/

    This reverts commit 1590de2d2a85506c29826c6914dc1cbe218d4113.

    Change-Id: I1acd257ea121e6ff48ce1605e9509e01ce6edb76
    Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index 8e9a292..ac60400 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit 8e9a292c2d50ee53eca470474700502b873a733f
+Subproject commit ac60400477b439a5cf8cdf77e990d2c0c8a75ab4