[leo_gateway][tool] Add conf test for customizing HTTP headers

[mocchira]

Background

We have a bit complicated conf file for http://leo-project.net/leofs/docs-old/admin_guide/admin_guide_11.html. So it would be good to have a feature to do conf test like nginx -t and also show ops friendly error messages that include what error (unexpected token, invalid time format and so on) happened at where (line number and token).

Links

Related: https://github.com/leo-project/leofs/issues/673 Souce: https://github.com/leo-project/leofs/blob/1.3.3-rc3/apps/leo_gateway/src/leo_nginx_conf_parser.erl