Allow proxy to be passed as a param

This is a similar change to https://github.com/stahnma/puppet-module-epel/commit/4d0c9197967cc66892d0ec474de0a910b0e05e26

Also fixes this puppet warning by adding inherits:

```
warning: Scope(Class[Atrpms]): Could not look up qualified variable 'atrpms::params::proxy'; class atrpms::params has not been evaluated at /tmp/vagrant-puppet/modules-0/atrpms/manifests/init.pp:18
```