interruptible_wait: allow nil Predicate
interruptible_wait: allow nil Predicate

Allow for the case where InterruptibleWait is used purely for the
timeout.
See the documentation string for details.

conceptboard/packer,dongyuzheng/packer-builder-xenserver,rdobson/packer-builder-xenserver,simonfuhrer/packer-builder-xenserver-1,june07/packer-builder-xenserver,fr34k8/packer-builder-xenserver,makunterry/packer-builder-xenserver,dongyuzheng/packer-builder-xenserver,xenserverarmy/packer,fr34k8/packer-builder-xenserver,rdobson/packer-builder-xenserver,makunterry/packer-builder-xenserver,june07/packer-builder-xenserver,xenserverarmy/packer,simonfuhrer/packer-builder-xenserver-1,conceptboard/packer