Add a wrapper script to check a named feature

For a given test name, run the JSON results parser and return an
error code that will be understood by Sensu.

It downloads the `results.json` file from the `pp-smokey` Jenkins job
artifact.

Example:
```
check_feature.sh backdrop_write
```

Would download the results of the last Jenkins pp-smokey run, parse the
JSON file and return 0 for success and nonzero for failure.
Make Jenkins output a results.json file

We can then use this as a build artifact. Sensu checks can download and
parse this.

Had to change the way we run from `bundle exec rake` to `bundle exec
cucumber` in order to pass arguments to cucumber.