Replace nginx::vhost with nginx::unicorn resource in server::unicorn class

Changes the configuration of the nginx vhost from using `nginx::vhost` to `nginx::unicorn`.
Removes the custom nginx vhost template in favour of specifying the configuration to the resource.
Supports using the `$servername` to define the SSL certificates instead of the `$::clientcert` if set.

The SSL Ciphers list may or may not be appropriate, I've based it on what I've been using for nginx recently.

This allows the nginx module to configure the vhost template instead of the puppet module, should mean the puppet module doesn't need to keep up to date with nginx configuration parameter forms.