State machine for the CeleryApplication status

[jmdacruz]

Need to keep track of the status of child objects (e.g., Deployments), and update the status of the CeleryApplication CRD accordingly (see zalando-incubator/kopf#268 and https://www.youtube.com/watch?v=rN_rQU92T5s&t=875s). Also, we should ideally assess the health of the cluster (at least right after it is created) and update the status of the CeleryApplication. Potential states:

• New: Right after the CRD is created, when the operator first reacts to it)
• Deployed: When all sub resources have been successfully created. For Deployments, this means that all ReplicaSets are at the desired number of Pods.
• Ready: When all celery workers are connected to the messaging broker and available to process tasks.