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
But thats not the way other module work. I just want to do 'include postfix' and everything else is done in hiera. If i would implemented it that way you suggest, i would also need to do 'include postfix::files'. I don't see any benefit here.