download: Handle the new scenario section in the deployment file

User can now specify a scenario section in their deployment file.
This commit will base the infra.yaml on the scenario specified,
or default to root infra.yaml file if nothing is specified.

This PR is based on the given layout for the openstak-yaml-infra repository

```
data/
    common.yaml.tmpl
    type.yaml.tmpl
    fqdn.yaml.tmpl
infra.yaml
heat.yaml
scenarios/
    3nodes/
         infra.yaml
         heat.yaml
     nonha/
         infra.yaml
         heat.yaml
```

**Note** : Default (ref-arch) is located at the root under the infra.yaml file, if you guys think it should be moved under scenarios/ref-arch let me know and I'll adapt the code accordingly.