Allow defining relationships in config schemas

Given the following piece of code in [`helper/schema/schema.go`](https://github.com/hashicorp/terraform/blob/e97b4803d678cb9154d377b49a968408bca422e6/helper/schema/schema.go#L344-399)

``` go
func (m schemaMap) Input(
input terraform.UIInput,
c *terraform.ResourceConfig) (*terraform.ResourceConfig, error) {
keys := make([]string, 0, len(m))
for k, _ := range m {
keys = append(keys, k)
}
sort.Strings(keys)

for _, k := range keys {
...
```

all config options are sorted alphabetically and treated independently in the prompt.
I can imagine cases where I would like the user to enter some config options first and eventually not ask for some other options based on the entered data.

Example?
Imagine following AWS provider config:
- `credentials_provider` (`detect | iam | env | static`)
- `credentials_file_path`
- `credentials_file_profile_name`
- `access_key`
- `secret_key`
- `security_token`

It is obvious that:
1. It would be annoying to ask user for all of these
2. Some options with certain values are making other ones superfluous (e.g. `credentials_provider` = `detect`, `iam` or `env` means that I should ignore all other options)

Would there be interest in such feature? If not, how would you suggest solving the problem with dependent config options above?

Related: hashicorp/terraform#1049

Start with helper/schema/schema.go, especially schemaMap.Input and the referenced lines that sort and prompt for options. Read the discussion and related hashicorp/terraform#1049 before attempting implementation. The issue proposes dependent configuration but does not define an accepted design or concrete done criteria.

Written by the indexing model from the issue text.