Make the module more generic and add CentOS support

This PR add the support for CentOS distribution (tested on CentOS 6.4). Instead of just add some bind::centos class, I did some refactoring to make the module much more generic. There is now no more need for a bind::debian class. The bind::base is generic and everything is defined in bind::params. Adding a basic support for a new Os should be as easy as add an entry in the params.pp and set the dozen of parameters accordingly to distribution configuration.

I also updated the tests so that it is not required to clone all the tests when a new distribution is added. a spec/defines/parameters.rb has been added with almost the same data than bind::params. All the _spec files then loop over the different distributions defined in parameters.pp and run all the tests for each distribution.

Maxence