Add ironic jobs to the CI

Ironic is now using diskimage-builder for producing and publishing
official production-ready IPA ramdisks. These jobs test ironic
against changes in diskimage-builder to ensure they still work:
* Two check-image jobs merely build an IPA image with DIB:
  * The CentOS 8 job is our primary now and should eventually
    become voting.
  * The CentOS 7 one is used on stable/train and will be removed
    once DIB stops supporting this version.
* The two other jobs build and use the IPA image to deploy a node
  in two different ways (pushed via iSCSI and pulled via HTTP).
  Since they're likely to be affected by factors outside of DIB
  control, they're added non-voting.

Depends-On: https://review.opendev.org/703585
Change-Id: Ibce594657bb90db1bac63fa9f965ee88586f8246

.zuul.d/project.yaml

@@ -28,6 +28,20 @@
         - dib-functests-bionic-python3-image
         - dib-functests-bionic-python3-extras
         - dib-functests-bionic-python3-containers
+        # IPA jobs that build an image without further validations.
+        # TODO(dtantsur): make the centos8 job voting once things stabilize
+        - ironic-python-agent-check-image-dib-centos8:
+            voting: false
+        # NOTE(dtantsur): this can be removed when DIB drops CentOS 7
+        # compatibility, but is useful for now since CentOS 7 is used in
+        # ironic Train CI.
+        - ironic-python-agent-check-image-dib-centos7:
+            voting: false
+        # IPA jobs that build AND validate an image via deployment (two ways).
+        - ipa-tempest-wholedisk-bios-ipmi-direct-dib-src:
+            voting: false
+        - ipa-tempest-partition-bios-ipmi-iscsi-dib-src:
+            voting: false
     check-arm64:
       jobs:
         - dib-functests-arm64-bionic