Update newrelic.cfg template to match template installed by New Relic
Fix license attribute in PHP Agent config newrelic.ini

Gotta use license_key. Likely fixes tests on CentOS!
Ensure we follow kitchen-vagrant docs on using vagrant-berkshelf
Refactor PHP Agent install/config to use provider & resource
Fix PHP Agent / app_monitoring attribute typo in README

Accurate READMEs make for happy cookbook users