Run ironic python agent image builds on trusty.

Force ironic python agent builds to run on trusty as these builds depend
on docker which depends on trusty.

Change-Id: Icbf193f4ad8450ce1f5f3ed652081ea3bd2b42e6

modules/openstack_project/files/zuul/layout.yaml

@@ -404,6 +404,9 @@ jobs:
   # Pecan ceilometer tests need new mongodb available in trusty
   - name: ^gate-pecan-tox-ceilometer-.*$
     parameter-function: set_node_options_default_trusty
+  # IPA image builds need docker which requires trusty
+  - name: ironic-python-agent-buildimage-coreos
+    parameter-function: set_node_options_default_trusty
   # tempest and d-g branchless tests need to only run on master
   # (needs to be early to not impact non-voting stanzas later)
   - name: ^.*(-icehouse|-havana)$