Split overlays into discrete functionality and test

- Closes #987

To make the overlays testable in isolation, and after discussion with
the TSC, removed host/time/source from overlay files.

Signed-off-by: Jonathon Anderson <janderson@ciq.com>

overlays/systemd.netname/rootfs/etc/systemd/network/10-ww4-netname.link.ww

@@ -0,0 +1,11 @@
+{{ range $devname, $netdev := .NetDevs -}}
+{{- $filename := print "10-ww4-netname-" $devname ".link" }}
+{{- file $filename }}
+# This file is autogenerated by warewulf
+{{- if and $netdev.Hwaddr $netdev.Device }}
+[Match]
+MACAddress={{$netdev.Hwaddr}}
+[Link]
+Name={{ $netdev.Device }}
+{{ end -}}
+{{ end -}}