Add backdrop write response to message on failure

When the backdrop smoke test fails to write to the write API add the
response to the Sensu error message.
Only set content type once in backdrop smoke test
Make backdrop smoke tests more robust

- Check the status code of the write API response
- Parse the write API response as JSON
- Check that the status is "ok"
- Rename some of the variables to be a little more consistent
Merge pull request #340 from alphagov/improve-backdrop-smoke-test

Improve backdrop smoke test