Provide a way for the operator to use an existing service if the name in the Service CR matches

[seansund]

Currently, the Service CR supports two distinct modes of operation:

1. If a plan is provided then the operator will provision a resource of the given name, service class, and plan
2. If the plan is Alias then the operator will look for an existing resource with the given name

It would be helpful if this behavior could be handled without requiring a value of Alias for the plan. E.g. When the operator reads the Service CR it looks for an existing service with the same name first and if one already exists uses that one instead of creating a new one. For backwards compatibility or to prevent unexpected behavior this new behavior can be controlled with an annotation, like use-existing