cloudfoundation/gke clusters unset reference values

Hiya.

given a config like so:

```
imports:
- path: templates/cloud-foundation/gke/gke.py
name: gke.py

resources:
- name: foobar
type: gke.py
properties:
zone: europe-west2-a
cluster:
name: foobar
initialClusterVersion: 1.12.5-gke.5
resourceLabels:
environment: production
# Enable logging
loggingService: logging.googleapis.com
intialNodeCount: 1
description: |
A new K8s Cluster
network: default
subnetwork: default
privateCluster: True
nodeConfig:
oauthScopes:
- https://www.googleapis.com/auth/compute
- https://www.googleapis.com/auth/devstorage.read_only
- https://www.googleapis.com/auth/logging.write
- https://www.googleapis.com/auth/monitoring
```

Creates the resource correctly, but then produces the following error:
```
ERROR: (gcloud.deployment-manager.deployments.update) Error in Operation [operation-1553790615076-5852a11aabfac-7a24be92-5a43673b]: errors:
- code: REFERENCE_VALUE_NOT_FOUND
location: /deployments/project/resources/foobar->$.properties
message: |-
The reference 'masterAuth.clientCertificate' is not found, reason:
The resource 'foobar' exists, but the reference value does not.
```