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-cache.yml

@@ -48,3 +48,8 @@ proxy:
   remoteurl: https://registry-1.docker.io
   username: username
   password: password
+health:
+  storagedriver:
+    enabled: true
+    interval: 10s
+    threshold: 3