Update git submodules

* Update docs/submodules/releng/builder from branch 'master'
  to 91624cf7eb0e7f97cf165ee0af011b459475b42a
  - Merge "Move devstack to common-packer vars/centos-7.json"
  - Move devstack to common-packer vars/centos-7.json

    Migrate to using the centos-7.json from CP for devstack so that
    we can stop maintaining the local centos.json var file.

    Change-Id: I721b73e6fef140984b2868c651f270c44174665d
    Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>

diff --git a/docs/submodules/releng/builder b/docs/submodules/releng/builder
index 1780688fdd5df75f4cd5c43d71b84e5feac4a205..91624cf7eb0e7f97cf165ee0af011b459475b42a 160000 (submodule)
--- a/docs/submodules/releng/builder
+++ b/docs/submodules/releng/builder
@@ -1 +1 @@
-Subproject commit 1780688fdd5df75f4cd5c43d71b84e5feac4a205
+Subproject commit 91624cf7eb0e7f97cf165ee0af011b459475b42a