Implement cluster ID

[oscarhermoso]

The cloudprovider interface expects a HasClusterID() method to be implemented, but currently this is hardcoded to return false.

var (
	_ cloudprovider.Interface = &Cloud{}
)

// ...

// HasClusterID returns true if a ClusterID is required and set
func (c *Cloud) HasClusterID() bool {
	return false
}

I'm not sure why a ClusterID is needed or what it should look like - raise a PR for this issue after kubernetes/kubernetes#128320 is closed