plugin/discovery: use refreshonly on the untar exec

Previously, if you change the image_name (such as for updating the
Foreman Discovery Image version), the new tar file would be downloaded
but not untarred because the content of the previous tar file would be
there and pass the `creates` check.

If we use `refreshonly` instead of `creates`, the untar exec will be
executed when the tar file is updated, as expected.

See also: #774 (comment)

manifests/plugin/discovery.pp

@@ -42,10 +42,10 @@
       remote_location => "${source_url}${image_name}",
       mode            => '0644',
     } ~> exec { "untar ${image_name}":
-      command => "tar xf ${image_name}",
-      path    => '/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin',
-      cwd     => "${tftp_root_clean}/boot",
-      creates => "${tftp_root_clean}/boot/fdi-image/initrd0.img",
+      command     => "tar xf ${image_name}",
+      path        => '/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin',
+      cwd         => "${tftp_root_clean}/boot",
+      refreshonly => true,
     }
   }
 }