Acceptance test should be able to run one specific test easily

A request coming from https://github.com/elastic/logstash/pull/5226#discussion_r61630135 ask for a very interesting point, having a simple option to run one single test. Acceptance test are very heavy test, so in case of debugging the whole process is faster for end user.

My current thoughts on implementing are going to the direction of using the filtering options from rspec [[examples matching](https://www.relishapp.com/rspec/rspec-core/v/3-4/docs/command-line/example-option)] [[group filtering](https://www.relishapp.com/rspec/rspec-core/v/3-4/docs/filtering)].

The interface should be in my opinion something like:

```
$ rake test:acceptance:example[example_string_name]
```

we could also enable in the feature to use the local `bin/rspec` shim with this test so the full rspec interface will be available.

Another interesting interface many people enjoy using is the one that lets examples to be run by line number. Do you find this interface also useful?

Start with the acceptance-test Rake entry point `test:acceptance:example` and the local `bin/rspec` shim, then review RSpec example and group filtering options. Done means a command can select and run one acceptance example without running the full suite; line-number filtering remains an interface option to evaluate.

Written by the indexing model from the issue text.