helpers: Further simplify the change url helper

[Josue-T]

The problem

In change url script we need workaround like this because path and domain are the old one and so somethings are set with old values.

Solution

Set correctly the domain and path with new values.

PR Status

Patch on helper was tested here: https://github.com/YunoHost-Apps/seafile_ynh/blob/master/scripts/change_url#L27-L52

[alexAubin]

I'm further simplifying the helper

The whole thing with the trick of "if only the domain change, we only mv the config" was supposedly design around the fact that the user may have manually modified the conf

Ironically, in this case we savagely overwrite the checksum, which means that next time ynh_add_nginx_config, the file will appear as not manually modified and be overwritten anyway

So let's reduce the complexity considering that this the current trick doesn't solve the problem anyway ...