Add and set Status Conditions 'Ready' in all resources

[ChunyiLyu]

Note to reviewers: remember to look at the commits in this PR and consider if they can be squashed

Summary Of Changes

• Per feedback from All resources have a minimum status #43, add a status condition to indicate that the CR was successfully created or updated by the controllers.
• Status condition 'Ready' looks like:

  conditions:
  - lastTransitionTime: "2021-02-23T20:52:21Z"
    status: "True"
    type: Ready
    reason: SuccessfulCreateOrUpdate
    message: ""

• 'Ready' status condition is set for all resources: queues, vhost, users, vhost, policies, exchanges, and bindings
• Extracted common error messages from controllers and created "controllers/common.go"

Additional Context

[coro]

Looks great, and I really like the error & conditions extraction.

This has made me think even more about the fact that a lot of our resources are rather similar. In broad strokes, create/update operations involve a declare through rabbithole, and delete operations require a delete. They all have a method to convert from k8s API JSON to rabbithole.XXXSettings{}. A lot of the controller reconcile code is copied between the objects.

I've been wondering whether this is a sign that we should have a common reconcile function, and use interfaces to provide the object-specific behaviours (which aren't very many). But possibly a refactor for another time.