For amazon ec2, use CreateFleet to launch spot instance instead of RequestSpotInstances

As mentioned in this AWS official document, RequestSpotInstances is a legacy API with no planned investment. It's recommended to use CreateFleet or CreateAutoscalingGroup to launch spot instances. I think CreateFleet is more suitable and simple for autoscaling runner.

To compare with RequestSpotInstances, CreateFleet is more flexible in allocation strategy and makes autoscaling runner more robust. capacity-optimized strategy can launch spot instances from the most available instance pools and set a priority for each instance type.

For testing this idea, I simply replaced RequestSpotInstances with CreateFleet API and upgraded aws-sdk-go to 1.44.70 in docker machine. My docker machine worked fine with autoscaling runner. It seems that CreateFleet is compatible with the current docker machine. Of course, I need to do more tests to make sure this changes won't break other functions.

Are you willing to make a schedule to adapt CreateFleet API for the amazon ec2 driver? I'm glad to implement this feature.