User friendliness: automatically adjust parameter lua/net.lua:front_mtu

The configuration file `lua/net.lua` comes with the parameters `front_mtu = 1500` and `back_mtu = 2048`, which is the proper setting for Gatekeeper servers, but not for Grantor servers. Grantor servers do not have a back net, and their front net should have an MTU of 2048.

Passing the parameter `gatekeeper_server` of `lua/main_config.lua` to `netf()` (i.e., the function that `lua/net.lua` returns) enables `lua/net.lua` to correctly set the parameters `front_mtu` accordingly to the role of the server.

Once this feature is implemented, the wiki page [Tips for Deployments](https://github.com/AltraMayor/gatekeeper/wiki/Tips-for-Deployments#grantor-server-configuration) must be updated to reflect this change.