distro/{rhel8,rhel84}: set systemd default targets

An image only had a systemd stage added if its blueprint contained
services or if its image type contained enabled services. The systemd
stage is now also added if the image type contains disabled services or
a default target.

The RHEL 8.4 qcow2 image type now specifies the multi-user default target.

In order to test this the image-info tool now includes the default
target in its output. Image test manifests are updated to include this
change.

test/data/manifests/rhel_8-ppc64le-qcow2-boot.json

@@ -10254,6 +10254,7 @@
         "version": "4.18.0-221.el8.ppc64le"
       }
     ],
+    "default-target": "graphical.target",
     "fstab": [
       [
         "UUID=0bd700f8-090f-4556-b797-b340297ea1bd",