Multiline option 'service_provider' for l2gw-plugin should not be put in a ini file

The option service_providers/service_provider now is put in a ini file l2gw-plugin.ini, which is inappropriate. Although there is no official standard for ini file format, normally, ini file shouldn't contain duplicate name in one section. The soft implementations are various in such case, it may cause issue when try to manipulate the file with other tools.

Changing the file name to l2gw-plugin.conf, might be a good idea.