Add clusters status in stage status

[maruina]

In #8 we introduced the concept of status.stages.

To complete the information about the status of each stage, we should extend status.stages with information about the cluster status.

It should look like

status:
  stages:
    - name: canary in emea
      phase: Completed
      message: Stage completed
      targets: 
        - cluster-1-eu-west-1a-1
        - cluster-2-eu-central-1b-1
      syncing: 
        - cluster-1-eu-west-1a-1
      requeued: []
      failed: []
      completed:
        - cluster-2-eu-central-1b-1
      startedAt: 2019-07-10T08:23:18Z
      finishedAt: 2019-07-10T08:40:18Z