installs to existing subnets when installconfig.platform.aws.subnets is specified

We can specify the ids for the subnet in the SubnetSpec: https://github.com/kubernetes-sigs/cluster-api-provider-aws/blob/main/api/v1beta2/network_types.go#L363

It is possible to specify other details, particularly the availability zone, but we suspect it is best for that to be unspecified and managed by the CAPA provider.