Added Hiera support for postfix::config, postfix::hash, postfix::transport and postfix::virtual

This PR adds support for Hiera. Some dependency were removed since Puppet throw dependency cycle errors:

```
Class['postfix']
```
I not really sure if this will throw errors in some edge cases, i found none in my tests.

```
Class['postfix'] -> Postfix::Hash <| |>
```
No longer needed since dependency to class{ 'postfix': } is managed in hash.pp
It can't be done in postfix::config since this is define type... My proposed way is the recommend way to implement hiera support.