Support a list of instance types in AWSManagedMachinePool

/kind feature

Describe the solution you'd like AWSManagedMachinePool has a field, instanceType, for specifying a single instance type. We should support a list of instance types that the underlying node group should use.

This is primarily useful for spec.capacityType: spot, where specifying a set of instances increases your odds of finding an adequate instance, but there's nothing stopping you from using it with spec.capacityType: onDemand. More here https://docs.aws.amazon.com/eks/latest/userguide/managed-node-groups.html#managed-node-group-capacity-types

See eks: allow specifying multiple instance types for AWSManagedMachinePool for the initial discussion. Thanks @jashandeep-sohi for starting this!

Anything else you would like to add: The implementation should wait for a launch template support for AWSManagedMachinePool.

• https://github.com/kubernetes-sigs/cluster-api-provider-aws/pull/3094

Environment:

• Cluster-api-provider-aws version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):