Adding an option to exclude all the tests tagged as integration

Since we can add multiple tags to a spec can we exclude a more generic tag? 
This will allow us to iterate on the plugins without updating this gem.

We could specify a test like this.
```ruby
describe LogStash::Output::Elasticsearch,  :integration => true, :docker => "elasticsearch"  do
end
```

And the two tags could be use to launch the appropriate docker? See ticket https://github.com/elasticsearch/logstash/issues/1737 for future work.