Configure port, owner user and service_name

I think your `system_install` solution is a bit magical and hackish. Adding an attribute to decide whether to manage the user seems to be a better solution and it does not add much complexity. At least its role is clear.

Something like the following:

```ruby
if node['owncloud']['manage_user']
  user # ....
  group # ...
end
```

This attribute should be `false` by default.