Restrict publishing jobs to master

We already build branched versions from IPA itself, let's not increase
our space usage.

Change-Id: I796b24a7ef19c3b0cc459ca1e1dc46d22f6349b5

zuul.d/project.yaml

@@ -45,6 +45,9 @@
     post:
       jobs:
         - publish-openstack-python-branch-tarball
-        - ironic-python-agent-build-image-tinyipa
-        - ironic-python-agent-build-image-dib-centos8
-        - ironic-python-agent-build-image-dib-debian
+        - ironic-python-agent-build-image-tinyipa:
+            branches: master
+        - ironic-python-agent-build-image-dib-centos8:
+            branches: master
+        - ironic-python-agent-build-image-dib-debian:
+            branches: master