Update git submodules

* Update docs/submodules/releng from branch 'master'
  - Merge "Releng PTL change to Trevor Bramwell"
  - Releng PTL change to Trevor Bramwell

    I am nominating myself as PTL of Releng following Fatih's announcement
    to step down.

    See this email thread for context:
    https://lists.opnfv.org/pipermail/opnfv-tech-discuss/2018-May/021142.html

    Change-Id: I6426a612378143d7829420cc9cf254021ce1eeae
    Signed-off-by: Trevor Bramwell <tbramwell@linuxfoundation.org>

diff --git a/docs/submodules/releng b/docs/submodules/releng
index f430b89..c771f89 160000 (submodule)
--- a/docs/submodules/releng
+++ b/docs/submodules/releng
@@ -1 +1 @@
-Subproject commit f430b896c1353887b6fa0b858950549bac3256f4
+Subproject commit c771f89e9cc24fd1b6cf5f775086acb5c3841e06