Dynamic YAML config by processing ERB first

The yaml configuration files are processed by ERB first. This allows
slightly more dynamic configs, such as support of environment variables
for vagrant vm/provisioner settings:

```yaml
---
  provisioner:
    - type: puppet
      manifest_file: <%= ENV['VAGRANT_MANIFEST'] || 'init.pp' >
```
Merge pull request #24 from nanliu/erb

Dynamic YAML config by processing ERB first