gluon-config-mode-hostname: fix site check for prefill

Do not require this setting in site.conf.

package/gluon-config-mode-hostname/check_site.lua

 need_boolean(in_site({'config_mode', 'hostname', 'optional'}), false)
-need_boolean(in_site({'config_mode', 'hostname', 'prefill'}), true)
+need_boolean(in_site({'config_mode', 'hostname', 'prefill'}), false)