Fixes#6447
This ensures that all variables of type string are consistently
converted to a string value upon running Terraform.
The place this is done is in the `Variables()` call within the
`terraform` package. This is the function responsible for loading and
merging the variables from the various sources and seems ideal for
proper conversion to consistent values for various types. We actually
already had tests to this effect.
This also adds docs that talk about the fake-ish boolean variables
Terraform currently has and about how in future versions we'll likely
support them properly, which can cause BC issues so beware.
James Bardin