Allow pre-definition of user passwords

[coro]

This PR allows a user to specify a Secret which contains a password to set on the User. The controller looks for the username & password under the keys username & password in the Secret. This Secret must be in the same namespace as the User, in order to mitigate privilege escalation.

Note that this import only happens upon creation - once the User is created, the import Secret is ignored. A user must be deleted & recreated in order to change a password or username until #35 is played.

This closes #36

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

Summary Of Changes

This PR adds several new fields to allow import of passwords from a created Secret:

apiVersion: v1
kind: Secret
metadata:
  name: credentials-secret
  namespace: testing
type: Opaque
data:
  username: bXktc2VjcmV0LXVzZXI=
  password: d2h5YXJleW91bG9va2luZ2hlcmU=
---
apiVersion: rabbitmq.com/v1alpha1
kind: User
metadata:
  name: import-user
  namespace: testing
spec:
  rabbitmqClusterReference:
    name: rmq
    namespace: rabbitmq-system
  importCredentialsSecret:
    name: credentials-secret

[coro]

cc @n3wscott as I believe this was one of the knative requirements.

[ChunyiLyu]

What we have discussed:

1. Import password and username from secret
2. Delete user.spec.name; if name is not present and no import secret provided, randomly generates username and password pair
3. two example manifests about how to create users: randomly generated username/password and pre defined password and username in docs/examples/users

[ChunyiLyu]

LGTM. There are several places in the controller which a failure was only returned but not logged or recorded as event. But I want to approve the PR so we can merge it soon and avoid merge conflicts. Logging and publish events for all errors can come in a different PR.