Extract scaling from zero

- extracting this package means it can be used in other components
such as the asynchronous nats-queue-worker which may need to
invoke functions which are scaled down to zero replicas.

Ref: https://github.com/openfaas/nats-queue-worker/issues/32



Tested on Docker Swarm for scaling up, already scaled and not
found error.

Signed-off-by: Alex Ellis (VMware) <alexellis2@gmail.com>