fedora-common-ostree: Make presets canonical

This ensures that only services defined in presets are enabled by
default. This has two advantages:
- services enabed in RPM scriptlets are dropped
- new services can be enabled by default simply by adding them to
  presets

This mirrors the behaviour of Fedora CoreOS. For more informations, see:
- https://github.com/coreos/fedora-coreos-config/blob/testing-devel/manifests/ignition-and-ostree.yaml#L39..L48
- https://github.com/coreos/fedora-coreos-config/pull/73
- https://github.com/coreos/fedora-coreos-config/pull/77
- https://github.com/coreos/fedora-coreos-config/pull/122
- https://github.com/coreos/rpm-ostree/issues/1803

fedora-common-ostree.yaml

@@ -111,4 +111,11 @@ postprocess:
     # Remove loader directory causing issues in Anaconda in unified core mode
     # Will be obsolete once we start using bootupd
     rm -rf /usr/lib/ostree-boot/loader
+
+    # Undo RPM scripts enabling units; we want the presets to be canonical
+    # https://github.com/projectatomic/rpm-ostree/issues/1803
+    rm -rf /etc/systemd/system/*
+    systemctl preset-all
+    rm -rf /etc/systemd/user/*
+    systemctl --user --global preset-all
 postprocess-script: "postprocess.sh"