sync: Add a new flag to enforce syncing only signed images, closes #455

sync: When checking if a image is already synced also check for changes in upstream signatures.

Signed-off-by: Petu Eusebiu <peusebiu@cisco.com>

examples/config-sync.json

@@ -22,6 +22,7 @@
 				"certDir": "/home/user/certs",
 				"maxRetries": 3,
 				"retryDelay": "5m",
+				"onlySigned": true,
 				"content":[
 					{
 						"prefix":"/repo1/repo",
@@ -64,4 +65,4 @@
 		]
 		}
 	}
-}
+}