Refactor to support Terraform 0.7

[likwid]

I wanted to start working on Terraform 0.7 #conversion, so I made a small set of changes, only refactoring to lists for a few variables (internal subnets, external subnets and availability zones).

I am trying to gauge interest in converting more of the stack automation over to Terraform 0.7 to use data sources, and native lists/maps where possible.

I wasn't sure if this was desirable or not, but figured I would test the waters.

[gregwebs]

The list hacks are pretty horrible, definitely a welcome improvement for me. I am using lists on my fork now and I see this warnings about not using data:

  * template_file.cloud_config: using template_file as a resource is deprecated; consider using the data source instead
  * template_file.policy: using template_file as a resource is deprecated; consider using the data source instead

[calvinfo]

I do like this, I haven't gotten a chance to review this in-depth but @yields is moving us to 0.7 internally next week. I think he can help review and ensure everything look good!