Add ImageRegistry field to Tenant Create and Tenant Update

[cesnietor]

Now both Update and Create Tenant allow the following field:

    ...
   "image_registry": {
        "registry": "https://index.docker.io/v1/",
        "username": "username",
        "password": "password"
    },
....

Which should create a Secret with the following fields:

{
  "auths" : {
    "https:\/\/index.docker.io\/v1\/" : {
      "username" : "username",
      "password" : "password",
      "auth" : "bWluaW9kZXY6ZmVjNzZjYTgtZjczMC00YWRmLWI4YTgtMDIwM2Y5234hlYjlj" # username:password encoded
    }
  }
}

apiVersion: v1
kind: Secret
metadata:
  name: minio-regcred-cred
  namespace: <namespace>
data:
  .dockerconfigjson: BASE64ENCODEDSECRET/BYTES
type: kubernetes.io/dockerconfigjson