This augments backend-config to also accept key=value pairs.
This should make Terraform easier to script rather than having to
generate a JSON file.
You must still specify the backend type as a minimal amount in
configurations, example:
```
terraform { backend "consul" {} }
```
This is required because Terraform needs to be able to detect the
_absense_ of that value for unsetting, if that is necessary at some
point.
df8529719c
