Add storagedriver section to health check configuration

Add default storagedriver health check to example configuration files
with parameters matching the previous hardcoded configuration.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

cmd/registry/config-example.yml

@@ -11,3 +11,8 @@ http:
     addr: :5000
     headers:
         X-Content-Type-Options: [nosniff]
+health:
+  storagedriver:
+    enabled: true
+    interval: 10s
+    threshold: 3