Move DIB from TripleO to Infra

The discussion was upstream:
http://lists.openstack.org/pipermail/openstack-dev/2017-January/110033.html

It seems like we've reached a consensus where DIB would be moved to
OpenStack Infra.

Indeed, most TripleO developers don't maintain DIB anymore while the
project is still active.
Also, there is an amount of overlap between the people working on DIB and on
general Infra-oriented tooling, so we're proposing to move DIB under
Infra umbrella.

Note: TripleO team is currently investigating how we can remove
dib-utils. See https://bugs.launchpad.net/tripleo/+bug/1673144
Change-Id: Ibd166de0069a332ed475c168beb5dd66beb9e90b

reference/projects.yaml

@@ -1249,6 +1249,9 @@ Infrastructure:
     devstack-gate:
       repos:
         - openstack-infra/devstack-gate
+    diskimage-builder:
+      repos:
+        - openstack/diskimage-builder
     elastic-recheck:
       repos:
         - openstack-infra/elastic-recheck
@@ -4697,9 +4700,6 @@ tripleo:
     dib-utils:
       repos:
         - openstack/dib-utils
-    diskimage-builder:
-      repos:
-        - openstack/diskimage-builder
     instack:
       repos:
         - openstack/instack