Always pause docker build jobs

The design idea for these jobs was that the buildset-registry job
would be used in systems where a dedicated registry job was desired
(e.g., because there are multiple image build jobs).  The image-build
job inherits from the buildset-registry job so that if a registry is
not already running, it runs one, but it overrides the playbook so
that it does not pause.  In other words, it was meant only to do what
is necessary to build its own image(s), not to serve them to other
jobs.

However, in the (arguably common) case that a repo has a single image
build job as well as jobs which could consume those images, it would
be efficient to have the image build job run the registry, build the
images, then pause for the registry to be used by the consuming jobs.

Rather than ask users to make a new playbook which restores the job
pause, let's have the image build job pause iff it is running a
buildset registry.  If there are no child jobs, Zuul will resume
the job immediately, so there won't be any change in behavior.  But
if there are child jobs, they will be able to use the registry
running on the image build job.

Change-Id: Iebd47305eeafbfaac0f699aae3e0a7d7a36f85ee

playbooks/docker-image/run.yaml

@@ -1,3 +1,16 @@
 - hosts: all
   roles:
     - build-docker-image
+
+# If buildset_registry is defined, that means a parent job is running it;
+# only if it is not defined does it mean that we are running it.  If we
+# are running it, pause the job so that child jobs will automatically
+# use it.
+- hosts: localhost
+  tasks:
+    - name: Pause the job
+      when: buildset_registry is not defined
+      zuul_return:
+        data:
+          zuul:
+            pause: true

zuul.yaml

@@ -230,6 +230,10 @@
 
       Analog of build-docker-image job, but with a buildset registry.
 
+      This job will pause after starting the registry so that it is
+      available to any jobs which depend on it.  Once all such jobs
+      are complete, this job will finish.
+
       .. include:: ../../playbooks/docker-image/README.rst
     run: playbooks/docker-image/run.yaml
     provides: docker-image