Add some missing smartos support to ohai

This PR is inspired by [the plugins in modcloth-smartos](https://github.com/modcloth-cookbooks/modcloth-smartos/tree/master/templates/default/plugins).

It adds memory and cpu checking for nodes and is required for instance by the [elasticsearch cookbook](https://github.com/elasticsearch/cookbook-elasticsearch/blob/master/attributes/default.rb#L43).

I'm probably doing something wrong here so I would like to hear back from you if my implementation is correct and what needs to be modified, I'll try to reimplement it better. 

Thank you !