REST API: Add event ID to list of chaos events

[mlafeldt]

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.

Before:

$ curl -s http://$DOCKER_HOST_IP:8080/simianarmy/api/v1/chaos | jq .[0]
{
  "monkeyType": "CHAOS",
  "eventType": "CHAOS_TERMINATION",
  "eventTime": 1462548706242,
  "region": "eu-west-1",
  "groupType": "ASG",
  "groupName": "some-group",
  "chaosType": "ShutdownInstance"
}

After:

$ curl -s http://$DOCKER_HOST_IP:8080/simianarmy/api/v1/chaos | jq .[0]
{
  "monkeyType": "CHAOS",
  "eventId": "i-1e0db592",
  "eventType": "CHAOS_TERMINATION",
  "eventTime": 1462548706242,
  "region": "eu-west-1",
  "groupType": "ASG",
  "groupName": "some-group",
  "chaosType": "ShutdownInstance"
}

[mlafeldt]

Looking at the broken test right now.

[mlafeldt]

Fixed the failing test. Accidentally also fixed another problem where the test checked for a field named instanceId, which doesn't exist.

Please take a look.

[ebukoski]

LGTM, thanks for the contribution!