introduce site overlays

site overlays are place in sysconfdir/overlays and take
precedence over distribution overlays with the same name.

Every `wwctl overlay` command changing overlays will create
an site overlay. distribution overlays can't be deleted or
modified with wwctl.

Signed-off-by: Christian Goll <cgoll@suse.com>

internal/pkg/configure/hostfile.go

@@ -14,7 +14,8 @@ import (
 Creates '/etc/hosts' from the host template.
 */
 func Hostfile() (err error) {
-	hostTemplate := path.Join(overlay.OverlaySourceDir("host"), "/etc/hosts.ww")
+	overlaySourceDir, _ := overlay.OverlaySourceDir("host")
+	hostTemplate := path.Join(overlaySourceDir, "/etc/hosts.ww")
 	if !(util.IsFile(hostTemplate)) {
 		return fmt.Errorf("'the overlay template '/etc/hosts.ww' does not exists in 'host' overlay")
 	}