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 
Makes sense.   I was following the lead of the above block but I'll fix that as well:  https://github.com/opscode/omnibus-supermarket/blob/irving/fix_system_dir/cookbooks/omnibus-supermarket/recipes/app.rb#L28-L29