Read private & public key paths from config

Read private_key and public_key from ~/.eyaml/config.yaml

```
private_key: '~/keys/eyaml/private_key.pkcs7.pem'
public_key: '~/keys/eyaml/public_key.pkcs7.pem'
```
Merge branch 'master' of https://github.com/sammcj/hiera-eyaml