auto-updates: Install and configure dnf-automatic

*dnf-automatic* is an add-on for `dnf` that performs scheduled,
automatic updates.  It works pretty much how I would want it to:
triggered by a systemd timer, sends email reports upon completion, and
only reboots for kernel et al. updates.

In its default configuration, `dnf-automatic.timer` fires every day.  I
want machines to update weekly, but I want them to update on different
days (so as to avoid issues if all the machines reboot at once).  Thus,
the _dnf-automatic_ role uses a systemd unit extension to change the
schedule.  The day-of-the-week is chosen pseudo-randomly based on the
host name of the managed system.

roles/dnf-automatic/defaults/main.yml

@@ -0,0 +1,4 @@
+dnf_automatic_reboot: when-needed
+dnf_automatic_email_from: root@example.com
+dnf_automatic_email_to: root
+dnf_automatic_email_host: localhost