Resolve "Terraform guest blog on GitLab CI/CD and TerraformPipelines - 2019-09-17"

Closes https://gitlab.com/gitlab-com/www-gitlab-com/issues/4954

Checklist for writer

• Link to issue added, and set to close when this MR is merged
• Due date and milestone (if applicable) added for the desired publish date
• If time sensitive
  • Added ~"priority" label
  • Mentioned @rebecca to give her a heads up ASAP
• Blog post file formatted correctly, including any accompanying images
• Review app checked for any formatting issues
• Reviewed by fellow team member
• Assign to @rebecca for review

/label ~"blog post"