[Feature] Update machinedeployments list dynamically

[rewiko]

Based on https://github.com/gardener/autoscaler/blob/machine-controller-manager/cluster-autoscaler/cloudprovider/mcm/mcm_cloud_provider.go#L155
Cluster-autoscaler with mcm provider do not implement autodiscovery of machinedeployments, Min, Max and autoscaling: true fields could be created in MCM to allow CA to get all machines deployment with autoscaling enabled.

Also when the static list is used, a check should be added make sure the machinedeployment exists. The refresh function will refresh the list from time to time.

For the moment if you define a machinedeployment as parameter to CA and this one do not exist the CA autoscaler will stop scaling the cluster even if some other machinedeployments are available for scaling.

[himanshu-kun]

Post grooming decision

We are going to add FAQ for a stand-alone usage to be aware of the issues that can arise.