Add Steps: Remove Getting Started panel from Grafana

#245 fixed a build issue in the Add Steps demo by upgrading Grafana. That version bump introduced a "Getting Started" panel in the Grafana dashboard, which should be removed.

## Background

Newer versions of Grafana include a "Getting Started" panel that should be disabled for our demo:

## Proposal

Change the anonymous org role from `Admin` to `Editor`:
```diff
diff --git a/add-steps/docker-compose.yml b/add-steps/docker-compose.yml
index b391b36..5573f71 100644
--- a/add-steps/docker-compose.yml
+++ b/add-steps/docker-compose.yml
@@ -149,7 +149,7 @@ services:
- 3000:3000
environment:
- GF_AUTH_ANONYMOUS_ENABLED=true
- - GF_AUTH_ANONYMOUS_ORG_ROLE=Admin
+ - GF_AUTH_ANONYMOUS_ORG_ROLE=Editor

baseline_slow_cooker:
image: buoyantio/slow_cooker:1.1.0-golang1.8rc2
```

Note that this breaks our `bootstrap.sh` script because we can no longer add datasources via the Grafana HTTP API:
```bash
$ curl http://localhost:3000/api/datasources
{"message":"Permission denied"}
```

To address this, add the Prometheus datasource via [Datasource provisioning](http://docs.grafana.org/administration/provisioning/#datasources) rather than via API. For example:
https://github.com/linkerd/linkerd-viz/blob/c6b4f2baff880ac5c093b6beaec1d42140ce2a48/linkerd-viz#L45-L61

Also bump the Kubernetes version to Grafana 5.4.3:
https://github.com/linkerd/linkerd-examples/blob/d50b911dec4d811a188f66979c45edc1fdb9a90e/add-steps/k8s/add-steps.yml#L918

Start with add-steps/docker-compose.yml and bootstrap.sh, then compare the referenced Grafana datasource provisioning example and add-steps/k8s/add-steps.yml. Run the Add Steps setup and verify that the dashboard no longer shows the Getting Started panel, the Prometheus datasource is provisioned, and Grafana access still works.

Written by the indexing model from the issue text.