Add php::fpm::pools parameter

The default www FPM pool will only be created if no pools are defined.
Depend on classes instead of resources with param titles
Validate php::fpm::pools only if used
FPM pools default to user and group set in php::fpm
Add documentation for php::fpm::pools parameter
Merge pull request #12 from Mayflower/fpm-fixes

FPM pools cleanups