Catalog Failure if Apache VHost has no priority

If configuring a farm to manage the VHost `SetHandler` fragment, an error occurs if the priority of that vhost isn't explicit in the resource definition.

```
Error: Evaluation Error: Error while evaluating a Resource Statement, Invalid tag '-servername.conf' (file: /etc/puppetlabs/code/modules/concat/manifests/fragment.pp, line: 38) (file: /etc/puppetlabs/code/modules/apache/manifests/vhost/fragment.pp, line: 74) on node servername
```

Maybe a new way to load this module should be considered, rather than this fragment.

Reproduce catalog evaluation for a farm managing the VHost SetHandler fragment without an explicit priority. Start with concat/manifests/fragment.pp at line 38 and apache/manifests/vhost/fragment.pp at line 74; done means this configuration catalogs without the reported Invalid tag error.

Written by the indexing model from the issue text.