This is the first step in allowing overrides of map and list variables.
We convert Context.variables to map[string]interface{} from
map[string]string and fix up all the call sites.
Adding walkValidate to the EvalTree operations, and removing the
walkValidate guard from the Interpolater.valueModuleVar allows the
values to be interpolated for Validate.
This commit adds support for declaring variable types in Terraform
configuration. Historically, the type has been inferred from the default
value, defaulting to string if no default was supplied. This has caused
users to devise workarounds if they wanted to declare a map but provide
values from a .tfvars file (for example).
The new syntax adds the "type" key to variable blocks:
```
variable "i_am_a_string" {
type = "string"
}
variable "i_am_a_map" {
type = "map"
}
```
This commit does _not_ extend the type system to include bools, integers
or floats - the only two types available are maps and strings.
Validation is performed if a default value is provided in order to
ensure that the default value type matches the declared type.
In the case that a type is not declared, the old logic is used for
determining the type. This allows backwards compatiblity with previous
Terraform configuration.
The context_test file has gotten pretty unruly. Let's split it up into
a few files so we can be nicer to our editors and our own sanity.
Definitely lots more we can do to clean up, but with changes like this
I'd rather do small, focused, clear steps instead of one big "cleaned up
lots of stuff" PR.
