Newrelic php agent: double config

- /etc/php5/conf.d/newrelic.ini: From our puppet module `newrelic::php5`
- /etc/php5/conf.d/20-newrelic.ini (symlink): From their apt package

How about changing `newrelic::php5` to use `php5::config_extension`? This way it will debian's `php5enmod` and then probably the apt-package detects that the config file is already installed etc.

@ppp0 can you look into this?