fix(config): Ignore any existing plugins stanzas. (spinnaker#1583)

Slack is littered with people having this issue. It looks like this was added in halyard 1.23 on 8/27/2019. So anyone with a config after that will have this issue.

halyard-config/src/main/java/com/netflix/spinnaker/halyard/config/model/v1/node/DeploymentConfiguration.java

@@ -87,6 +87,9 @@ public class DeploymentConfiguration extends Node {
 
   Canary canary = new Canary();
 
+  // For backwards compatibility, removed in spinnaker/halyard#1520
+  @JsonIgnore String plugins = "";
+
   Spinnaker spinnaker = new Spinnaker();
 
   Webhook webhook = new Webhook();