Design: CreatableImpl should not assume it's Resource

[anuchandy]

CreatableImpl represents ANY type that can be created not only the types that implements Resource. https://github.com/Azure/azure-sdk-for-java/blob/master/azure-mgmt-resources/src/main/java/com/microsoft/azure/management/resources/fluentcore/model/implementation/CreatableImpl.java#L32

The below typecast (Creatable<? extends Resource>) this results null for the create-able type that does not extends from Resource.

    protected CreatableImpl(String name, InnerModelT innerObject) {
        super(name, innerObject);
        creatableTaskGroup = new CreatableTaskGroup(name, (Creatable<? extends Resource>) this, this);
    }

ResourceImpl is one type that inherits from CreatableImpl that does not mean thats the only one.

[anuchandy]

addressed in #973