[WiP] RSpec improvements and upgrade to RSpec 3.0

This is a working progress PR, initially started with the motivation in mind to provide clarity to the LogStash tests and to upgrade them to 3.0

This is an WiP PR, that is mainly open so we can reach a common point on how do we want to have our test looking like. 

If you have any question or doubt about why something is like it is, fire a comment :smile: 

- purbon
looking at the new file as a whole I find it difficult to understand what the tests are doing, since a lot of the setup for the tests is in the fixtures file. perhaps I'm not getting the whole idea of this.