Add varnish_param resource type and providers

This PR adds a new `varnish_param` resource type.

It allows to modify varnish parameters where OSes put them by default:

* in `/etc/default/varnish` for Debian wheezy (as a list of parameters)
* in `/etc/systemd/services/varnish.service` for Debian jessie (as a list of parameters)
* in `/etc/sysconfig/varnish` for RedHat 6 (as standalone variables)
* in `/etc/varnish/varnish.params` for RedHat 7 (as standalone variables)

Example:

```puppet
varnish_param { 'listen_address':
  ensure => present,
  value   => 'localhost',
}
```