Add cookbooks, role and cloudformation template to support deploying Platform from Nexus

This PR adds the following:
- `nexus-artifact` cookbook, providing helpers to generate URLs for retrieving Nexus artifacts
- `signifyd-platform` cookbook, install signifydplatform from Nexus and configure to run as a service
- `signifyd-platform` role which invokes the above `signifyd-platform` cookbook
- `platform-from-nexus` CloudFormation template which builds a stack using the `signifyd-platform` role

Both cookbooks include scaffolding for ChefSpec and Test Kitchen cookbook testing. The cookbooks and roles included here are already uploaded to the Chef Server and ready for testing with the `platform-from-nexus` CloudFormation template.

Once this `platform-from-nexus` template becomes the basis for production deployment, the `signifyd-platform` role will supersede the existing `platform`, `base`, `staging`, and `production` roles.