Multiple class support

[kfox1111]

There are reasons to support multiple spire clusters within a k8s cluster which are configured with spire-controller manager. To support this, two new options should be added.

1. ClassName. This functions like ingress / storage where you can specify on the CR's which class should handle the objects.
2. MissingClassName, a setting to tell the controller how to behave with CR's that don't have a Class Name.

Not setting either would have the traditional behavior.