Add port for ynh_add_nginx_config

[Krakinou]

Problem

• When __ PORT__ is used in nginx.conf, ynh_add_nginx_config will not changed it if $port is not initialized with the port number. This will cause nginx to fail at reload.

Solution

• Initialize $port from the app settings. If port is not set in app settings, nothing will happen

PR Status

• Code finished.
• Tested with Package_check.
• Fix or enhancement tested.
• Upgrade from last version tested.
• Can be reviewed and tested.

[yalh76]

There is no __PORT__ in the nginx.conf provided in example_ynh, so that could be strange for a new developper...
In addition, in that case $port initialisation is missing in the upgrade script.

For me, example_ynh should be as generical as possible without implementing all possibilities but providing a good package start and main guidelines

[ericgaspar]

@Krakinou as a point. It is confusing not to mention $port variable... this variable should at least appear as a commented option. (like the database variables L.32-35) I encountered several time errors in the change_url script before realizing that the port variable was missing ...

[alexAubin]

(As a followup, related to YunoHost/yunohost#1118 , maybe ynh_add_config could report an error if it found some __FOOBAR__ to be replaced but no corresponding variable to replace it with)

[yalh76]

Now ynh_add_config report an error when an argument is missing :)