Added convention based defaults and support for setting base path in API Gateway custom domain

[eric-swann-q2]

Hello @jgilbert01
First, thanks for this great project, it helped me a lot. I made several adjustments and figured I'd submit them to you for consideration. I believe that the existing configuration approach should continue to work though. I wanted the exact thing you had here but wanted to pull settings where possible rather than specify them and derive most things from a few settings. I've made the following additions:

• Derive most settings from the domain name, including regional domain names and certificate ARNs if a specific ARN is not specified
• Create default health checks if no health check ID's are specified
• Derive the naming of regional domain names from the default domain name
• Create a dummy API definition so that the base path can be set in the API Gateway Custom domain instead of having to add a path in the Cloud Formation distro.

If these changes don't work for you, not a big deal, but thought I'd submit the work I've done in case any of these things help. Thanks again for the plugin, it helped me immensely.