[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
Now that insist is not being used here, this dependency can be removed here and within logstash-devutils here:
https://github.com/elasticsearch/logstash-devutils/blob/master/lib/logstash/devutils/rspec/spec_helper.rb#L3