Install yum-plugin-priorities on centos images

When installing from dlrn repos on centos, we need to respect the
repo priorities.  This is not needed on rhel, so it is added to the
centos-specific image file only.

Change-Id: Idd083a4bf6f4cf11c8f552c4ff9fea05a3c8b79f

image-yaml/overcloud-images-centos7.yaml

@@ -8,6 +8,8 @@ disk_images:
       - selinux-permissive
     environment:
       FS_TYPE: xfs
+    packages:
+      - yum-plugin-priorities
   -
     imagename: ironic-python-agent
     arch: amd64
@@ -15,4 +17,6 @@ disk_images:
     distro: centos7
     elements:
       - selinux-permissive
+    packages:
+      - yum-plugin-priorities