added support for metrics reporter

This adds by default the graphite metrics reporter library as it is the most common usecase.
In future one should be able to add several additional libraries, but for now this will do.
`hash_to_yaml_string` is executed before the `only_if` condition is checked, which results in this error when the config is nil:

```
================================================================================       
Recipe Compile Error in /tmp/kitchen/cookbooks/cassandra/recipes/default.rb       
================================================================================       
       
NoMethodError       
-------------       
       undefined method `to_hash' for nil:NilClass
       
       Cookbook Trace:
       ---------------
         /tmp/kitchen/cookbooks/cassandra/libraries/config_helpers.rb:19:in `hash_to_yaml_string'
         /tmp/kitchen/cookbooks/cassandra/recipes/datastax.rb:191:in `block in from_file'
       
         /tmp/kitchen/cookbooks/cassandra/recipes/datastax.rb:184:in `from_file'
         /tmp/kitchen/cookbooks/cassandra/recipes/default.rb:20:in `from_file'
       
       Relevant File Content:
       ----------------------
       /tmp/kitchen/cookbooks/cassandra/libraries/config_helpers.rb:
       
        12:      return 'true'
        13:    else
        14:      return 'false'
       
        15:    end
        16:  end
        17:  
        18:  def hash_to_yaml_string(hash)
        19>>   return hash.to_hash.to_yaml
        20:  end 21:  
```