refactored overlay class

overlay.GetOverlay(name) returns now an error if the overlay doesn't
exist. This is the most canonical way to act if there is no overlay.

internal/pkg/configure/hostfile.go

@@ -14,7 +14,10 @@ import (
 Creates '/etc/hosts' from the host template.
 */
 func Hostfile() (err error) {
-	overlay_ := overlay.GetOverlay("host")
+	overlay_, err := overlay.GetOverlay("host")
+	if err != nil {
+		return err
+	}
 	hostTemplate := path.Join(overlay_.Rootfs(), "/etc/hosts.ww")
 	if !(util.IsFile(hostTemplate)) {
 		return fmt.Errorf("'the overlay template '/etc/hosts.ww' does not exists in 'host' overlay")