Manage apt::holds in hiera, like apt::sources

It's nice to be able to manage holds with hiera as well, just like sources.

Usage:

```yaml
apt::holds:
  facter:
    version: 2.3.*
  hiera:
    version: 1.3.*
  puppet:
    version: 3.7.*
  puppet-common:
    version: 3.7.*
```