Add the ability to specify a custom pool template file.

@Slashbunny phpfpm looks fantastic and I thank you for sharing it.  You've saved me a huge amount of work.

I think it might be useful to be able to override the default template used for the worker pool conf file, though.  With that in mind, I've added the $pool_template_file parameter to params.pp, with a default value to match the current 'phpfpm/pool.conf.erb'.  The phpfpm::pool now uses that parameter, so it could be overridden, as in the example I added to the README.md.

Is this a contribution you would interested in accepting?  If so, what should I change?  Is $pool_template_file the parameter name we should use, or should it be just $pool_template?

Thanks,

Tim