helm chart: updating images/tags w the release (0.20.0)

[markmark206]

This change goes together with temporalio/temporal#244

The images of this release are tagged 0.20.0. Updating helm chart to reference this release's images.

I tested this by deploying to a test k8s cluster and making sure the deployment completed, and that the images that are running is the version that I expect.

Example:

1. install helm chart

~/src/temporal-helm-charts $ helm install temporaltest .
NAME: temporaltest
LAST DEPLOYED: Sat Mar 21 15:39:57 2020
NAMESPACE: default
STATUS: deployed
REVISION: 1
NOTES:
To verify that Temporal has started, run:

  kubectl --namespace=default get pods -l "app.kubernetes.io/instance=temporaltest"

2. double-check image tags

~/src/temporal-helm-charts $ kubectl get pods
NAME                                                    READY   STATUS    RESTARTS   AGE
...
temporaltest-history-657c6d6648-fcf6l                   1/1     Running   2          6m19s
...

~/src/temporal-helm-charts $ kubectl describe pods/temporaltest-history-657c6d6648-fcf6l | grep "0.20.0"
    Image:          temporalio/server:0.20.0
  Normal   Pulling    12m                kubelet, ip-10-0-3-34.us-west-2.compute.internal  Pulling image "temporalio/server:0.20.0"
  Normal   Pulled     12m                kubelet, ip-10-0-3-34.us-west-2.compute.internal  Successfully pulled image "temporalio/server:0.20.0"
  Normal   Pulled     12m (x2 over 12m)  kubelet, ip-10-0-3-34.us-west-2.compute.internal  Container image "temporalio/server:0.20.0" already present on machine