Parallel and sequential feature for the node scenario

[agabriel81]

Adds a new feature on the node scenario in order to stop/start multiple nodes (with the same or different labels) in parallel.
Example of stop/start multiple (7) nodes in parallel within the same availability zone (us-east-1), same label:

node-outage:
  action: node_stop_start_scenario
  label-selector: 'node-role.kubernetes.io/zone=us-east-1'
  instance-count: 7
  runs: 1
  parallel: true
  cloud-type: aws
  duration: 60

Example of stop/start multiple (2) nodes in parallel belonging to different labels:

node-outage:
  action: node_stop_start_scenario
  label-selector: [workerpool=stateless, workpool=stateful]
  instance-count-per-label: 2 
  runs: 1
  parellel: true
  cloud-type: aws
  duration: 60