[COOK-3322] Ohai plugin does not work with source install method

http://tickets.opscode.com/browse/COOK-3322

source recipe rewrite node['nginx']['binary'] 

```ruby
node.set['nginx']['binary']          = node['nginx']['source']['sbin_path']
```

and ohai_plugin use it, 

```ruby
  variables(
    :nginx_prefix => node['nginx']['source']['prefix'],
    :nginx_bin => node['nginx']['binary']
  )
```

```erb
status, stdout, stderr = run_command(:no_status_check => true, :cwd => "<%= @nginx_prefix %>", :command => "<%= @nginx_bin %> -V")
```

so <code>nginx::source</code> recipe must be included before <code>nginx::ohai_plugin</code>.