Service wait for Kubernetes native pods

To support clean deployments and correct depedency management, the quarks-operator allows a Kubernetes Pod to wait until one (or more) Service is available.

The operator does that by injecting an InitContainer which waits for the service to be up.

This is a generalization of the serialization hints natively available to all BOSH deployments.

The Pods needs to have the quarks.cloudfoundry.org/wait-for annotation, for example:

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apiVersion: v1
 kind: Pod
 metadata:
   annotations:
     quarks.cloudfoundry.org/wait-for: '[ "nats-headless" , "nginx" ]'

At the ops level this is achieved by an instruction like

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- type: replace
  path: /instance_groups/name=THE_INSTANCE_GROUP/env?/bosh/agent/settings/annotations/quarks.cloudfoundry.org~1wait-for
  value: '[ "uaa" ]'

The env/bosh/agent/settings/annotations key is a hash used by the operator to add additional annotations to the k8s objects it creates for an instance group. IOW they are applied to the generated quarks-statefulset, statefulset, and pod.

:warning: Note that while the dependency information is ultimately processed as a json array of strings, at the level of the annotations it has to be specified as a plain string. Just one which contains proper json syntax.

If full custom dependencies are not required, just (partial) serial startup of instance groups (in the order of their specification in their deployment) then the native serialization hints are likely good enough.

They are specified via a construction of the form

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instance_groups:
- name: THE_INSTANCE_GROUP
  update:
    serial: true

in the BOSH deployment, if under direct control, or via an ops file like

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- type: replace
  path: /instance_groups/name=THE_INSTANCE_GROUP/update/serial
  value: true

if the deployment cannot be modified directly.