Fix and return the CentOS 7 job

This job is used on stable branches and is currently broken. This
change introduces a workaround for older DIB and re-enables the job.

Change-Id: I100576c639649a69a8e95463ca87bedda4bbadc3

.zuul.yaml

@@ -127,6 +127,8 @@
         - ipa-tempest-wholedisk-bios-ipmi-iscsi-tinyipa-src
         - ironic-python-agent-check-image-tinyipa
         - ironic-python-agent-check-image-dib-centos8
+        - ironic-python-agent-check-image-dib-centos7:
+            voting: false
         - ironic-python-agent-check-image-dib-fedora:
             voting: false
         - ironic-python-agent-check-image-dib-ubuntu:

roles/ipa-build-dib-image/tasks/install.yaml

@@ -25,3 +25,14 @@
     name: "{{ ansible_user_dir }}/{{ zuul.projects['opendev.org/openstack/diskimage-builder'].src_dir }}"
   become: true
   when: dib_from_source
+
+# NOTE(dtantsur): work around the issue in older DIB versions when some
+# elements try to use the default Python instead of the one DIB is using,
+# failing with "No module named yaml"
+- name: Install PyYAML in Python 2
+  pip:
+    name: PyYAML
+    extra_args: -c "{{ ansible_user_dir }}/{{ zuul.projects['opendev.org/openstack/requirements'].src_dir }}/upper-constraints.txt"
+    executable: pip2
+  become: true
+  ignore_errors: true