Add the possibility to customize package name.

For example, this add the possibility to install 'collectd-core' instead 'collectd'. Less dependencies (no consolekit etc ...) and enough to make the collector work on clients. Tested on Debian.

Thank you in advance.
I think you need the class to inherits collectd::params instead of including it. If you do that then you don't need the pick(real_package_name) stuff