Add Authorization support

[wallyqs]

This adds support for configuring users and permissions on subjects on a NatsCluster. The users permissions have to be defined as a Secret in JSON which is then included as part of the configuration of the server:

kubectl create secret generic nats-clients-auth --from-file=clients-auth-permissions.json

Then as part of the NatsCluster definition, it is specified what is the secret from where the users permissions will be picked up:

apiVersion: "nats.io/v1alpha2"
kind: "NatsCluster"
metadata:
  name: "example-nats-auth"
spec:
  size: 3
  version: "1.0.4"

  auth:
    # Definition in JSON of the users permissions
    clientsAuthSecret: "nats-clients-auth"

Fixes #1
Fixes #31

[wallyqs]

since gnatsd v1.1.0 is out, could wait for the Docker image to be available and remove all of this and just use JSON here (filed issue to remove: #39)

[ColinSullivan1]

LGTM with a visual pass through. I agree with the thought you mentioned in slack about waiting for the gnatsd v1.1.0 docker image for JSON support.