REST API: Add event ID to list of chaos events

In order to get the IDs of terminated EC2 instances after the fact.

With this change, both `POST` and `GET` will return the same JSON fields
for each event. For some reason, `eventID` (the EC2 instance ID) was
missing when retrieving events via the REST API.

Author: mlafeldt

src/main/java/com/netflix/simianarmy/resources/chaos/ChaosMonkeyResource.java

@@ -137,6 +137,7 @@ public Response getChaosEvents(@Context UriInfo uriInfo) throws IOException {
         for (Event evt : evts) {
             gen.writeStartObject();
             gen.writeStringField("monkeyType", evt.monkeyType().name());
+            gen.writeStringField("eventId", evt.id());
             gen.writeStringField("eventType", evt.eventType().name());
             gen.writeNumberField("eventTime", evt.eventTime().getTime());
             gen.writeStringField("region", evt.region());