Add the ability to specify a custom template file for the main FPM confi...

@Slashbunny Thanks for accepting my previous pull request.  Now that we can specify a custom template for the worker pool conf file, how about allowing the main php-fpm.conf file to be generated from a custom template?

Again, not certain if the parameter name should be different, but having _file at the end matches what was done for the worker conf template.

Feedback appreciated.