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>

docs/man/man5/defaults.conf.5

@@ -18,9 +18,24 @@ structure of a node within \fBnodes.pp\fP.
 .EX
 defaultnode:
   runtime overlay:
-  - generic
+  - hosts
+  - ssh.authorized_keys
+  - syncuser
   system overlay:
   - wwinit
+  - wwclient
+  - fstab
+  - hostname
+  - ssh.host_keys
+  - issue
+  - resolv
+  - udev.netname
+  - systemd.netname
+  - ifcfg
+  - NetworkManager
+  - debian.interfaces
+  - wicked
+  - ignition
   kernel:
     args: quiet crashkernel=no vga=791 net.naming-scheme=v238
   init: /sbin/init