Default to spread worker nodes across failure domains

[richardcase]

trafficstars

/kind feature

Describe the solution you'd like Currently, CAPI will spread control plane machines across the reported failure domains (i.e. availability zones). It doesn't do this for worker nodes, machines in a machine deployment (or machines on their own).

Current advice is to create separate machine deployments and manually assign an az (via FailureDomain) to each of the machine deployments to ensure that you have worker machines in different azs.

It would be better when creating machines (if no failure domain is specified on the Machine) that we use the failuredomains on the Cluster and create the machine in a failure domain with the least amount of machine already. CAPI has some functions we could potentially use. Something like this:

machines := collections.FromMachineList(machinesList)
failureDomain := failuredomains.PickFewest(m.Cluster.Status.FailureDomains, machines)

Anything else you would like to add: We need to investigate if this is feasible, or if it is something that should be upstream in machine deployments.

Environment:

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