Add filed Pod to StepActions

[harryge00]

Marathon supports Pod now. And in the v1.7.0 events, App can be omitted if StepAction only includes Pod .

[harryge00]

event: deployment_step_success
data: {"plan":{"id":"38d05398-3024-4bb4-82ec-93fbb6534485","steps":[{"actions":[{"action":"StartPod","pod":"/simplepod23"}]},{"actions":[{"action":"ScalePod","pod":"/simplepod23"}]}],"version":"2018-12-26T09:11:03.950Z"},"currentStep":{"actions":[{"action":"ScalePod","pod":"/simplepod23"}]},"eventType":"deployment_step_success","timestamp":"2018-12-26T09:11:05.494Z"}

event: deployment_success
data: {"id":"38d05398-3024-4bb4-82ec-93fbb6534485","plan":{"id":"38d05398-3024-4bb4-82ec-93fbb6534485","steps":[{"actions":[{"action":"StartPod","pod":"/simplepod23"}]},{"actions":[{"action":"ScalePod","pod":"/simplepod23"}]}],"version":"2018-12-26T09:11:03.950Z"},"eventType":"deployment_success","timestamp":"2018-12-26T09:11:05.496Z"}