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

cmd/registry/config-dev.yml

@@ -59,4 +59,8 @@ notifications:
           threshold: 10
           backoff: 1s
           disabled: true 
-
+health:
+  storagedriver:
+    enabled: true
+    interval: 10s
+    threshold: 3

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