Make alert expiration time configurable

[Thomas Obenaus]

Currently it's hardcoded.
See: https://github.com/ThomasObenaus/sokar/blob/master/scaleAlertAggregator/scaleAlertPool.go#L46

[Serhii Kashuba]

Related alertmanager configuration:

  - receiver: receiver_name
    match:
      type: autoscaling
      component: CMP
    repeat_interval: 60s  # Equal to the expiration interval
    group_interval: 10s   # Less than repeat_interval

[Thomas Obenaus]

Done
Docker image version docker pull thobe/sokar:v0.0.4-6-gd9ddcc5