Display correctly ENV values containing spaces

Command `dokku config <app>` doesn't return entire env values (only to the first space) e.g.:

```
$ cat ENV
export ALLOWED_HOSTS='domain.com test.domain.com dev.domain.com'
export NEXT_VARIABLE=value

$ dokku config <app>
ALLOWED_HOSTS:    "domain.com
NEXT_VARIABLE:    value
```