Add options for pm for PHP-FPM execution mode

This forum post suggested making the FPM configurable:

Seems like a reasonable idea to me, and probably not complicated. I'd probably suggest handling pm, pm.max_children and min/max_spare_servers.



I don't think we can have the kind of setup suggested in that forum post (zero continually running tasks), as each domain needs it's own FPM pool that runs as it's own user.

I'm using the ondemand pm for alot of websites right now, and it's working like a charm. Probably missing something here, but the 'ondemand' pm saves a ton of ram compared to the default 'dynamic' FPM config.