amazonec2 driver should not use spot blocks

[flavioleggio]

As stated on one of the latest deprecation notices from AWS and documented here

Spot Instances with a defined duration (also known as Spot blocks) are no longer available to new customers from July 1, 2021. For customers who have previously used the feature, we will continue to support Spot Instances with a defined duration until December 31, 2022.

For what I understand from the amazonec2 driver documentation, this driver only supports spot instances in block mode. In order to optimize costs with some use cases for non-production workloads (batch processes, automated build systems and so on) it would be nice if standard spot instances were supported too.