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.
Fixes#10716
This trims whitespace around the key in the `-var` flag.
This is a regression from 0.7.x.
The value is whitespace sensitive unless double-quoted. This is the same
behavior as 0.7.x. I considered rejecting whitespace around the '='
completely but I don't want to introduce BC and the behavior actually
seems quite obvious to me.
