Update git submodules

* Update docs/submodules/releng from branch 'master'
  - Merge "Write the right default values in DOCKER_TAG"
  - Write the right default values in DOCKER_TAG

    It avoids facing with wrong substitutions [1]

    [1] https://build.opnfv.org/ci/job/functest-apex-baremetal-daily-gambia/23/console

    Change-Id: I4b8c990add51024d7984cd10668a33dd7c50c395
    Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index d2b0f38..a7108b1 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit d2b0f38a91aecd4259a3b060553d1429ec5babe3
+Subproject commit a7108b14b43f951da3cb1bb3d6ea5305642ba9ee