Use rest.Config for discovery URL instead of env var

- Why? Because the discovery URL is already there in the kubeconfig; let's
  not make our lives more complicated by passing it in via an env var.
- Also allow for ytt callers to not specify data.values.discovery_url - there
  are going to be a non-trivial number of installers of placeholder-name
  that want to use the server URL found in the cluster-info ConfigMap.

Signed-off-by: Andrew Keesler <akeesler@vmware.com>

deploy/deployment.yaml

@@ -25,7 +25,7 @@ data:
   #@yaml/text-templated-strings
   placeholder-name.yaml: |
     discovery:
-      url: (@= data.values.discovery_url @)
+      url: (@= data.values.discovery_url or "null" @)
     webhook:
       url: (@= data.values.webhook_url @)
       caBundle: (@= data.values.webhook_ca_bundle @)