Update git submodules

* Update docs/submodules/releng from branch 'master'
  - Set empty strings in DOCKER_TAG by default

    It let functest-alpine process branches right after [1].

    [1] https://build.opnfv.org/ci/job/functest-apex-virtual-suite-master/241/console

    Change-Id: I6ed804fe9f3628f67421157433e09da3bcbf585c
    Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
    Signed-off-by: Tim Rozet <trozet@redhat.com>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index 2b8a308..082d942 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit 2b8a30872c4d1e5fe9dbe310ba206522c02115b8
+Subproject commit 082d942902c4d822d64affaa3c0ce0bc9e1d1774