Fix issue where cookbooks cannot be uploaded

Supermarket (the rails application) is configured to upload cookbook tarballs to `/opt/supermarket/embedded/service/supermarket/public/system` but this directory doesn't exist on the filesystem and rails cannot create it:

```
D, [2014-12-10T23:02:51.679729 #10843] DEBUG -- :    (0.2ms)  ROLLBACK
I, [2014-12-10T23:02:51.680454 #10843]  INFO -- : Completed 500 Internal Server Error in 173ms
F, [2014-12-10T23:02:51.682131 #10843] FATAL -- : 
Errno::EACCES (Permission denied @ dir_s_mkdir - /opt/supermarket/embedded/service/supermarket/public/system):
  app/models/cookbook.rb:266:in `block in publish_version!'
  app/models/cookbook.rb:249:in `publish_version!'
  app/models/cookbook_upload.rb:37:in `block in finish'
  app/models/cookbook_upload.rb:36:in `tap'
  app/models/cookbook_upload.rb:36:in `finish'
  app/controllers/api/v1/cookbook_uploads_controller.rb:47:in `create'
```

This PR symlinks public/system to the var_directory. 
CC: @smith @cwebberOps @raskchanky 
I think it would be better to define a node['supermarket']['data_directory'] that defaults to the same value as this. That would make it easier to point it at an existing mounted volume and would make it so the leaf directory did not have to be called "supermarket_data."