Update git submodules

* Update docs/submodules/releng from branch 'master'
  - Merge "dovetail: docker tag support"
  - dovetail: docker tag support

    JIRA: DOVETAIL-419

    since dovetal has no A/B/C/D branch, only have master,
    need docker image name sth like opnfv/dovetail:plugfest_april_2017
    rather than opnfv/dovetail:<release>.1.0, etc.
    then when RELEASE_VERSION is empty, generate opnfv/dovetail:latest
    ortherwise opnfv/dovetail:<RELEASE_VERSION>

    Change-Id: I92573bee35da16305074393401e5e5da65aaa22c
    Signed-off-by: MatthewLi <matthew.lijun@huawei.com>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index 0f83643..5a2e5ed 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit 0f836439dcadba052b1c7e6751f4346ac256a2f1
+Subproject commit 5a2e5ed064cd98daafac512febc4dab36f67c656