Make it possible to verify whether resources were updated or not

[aermakov-zalando]

It's impossible to figure out whether the controller has processed the changes and updated the resources or not. The following information needs to be exposed so it's possible to figure out if the updates are done or still in progress:

• observedGeneration in Stackset.status, updated only after the changes to the stacks are done
• observedGeneration on Stack.status, updated only after the subresources have been changed
• currentStackName in Stackset.status to avoid duplicating the weird default and name generation logic

[mikkeloscar]

Somewhat related to #49

[aermakov-zalando]

It would also be nice if Status reflected the state of the subresources as well (e.g. Ingress if it's necessary).