[autoscaler] Add an initial_workers option (#3530)

## What do these changes do?

    This option goes along with `min_workers`, and `max_workers`.  When the
    cluster is first brought up (or when it is refreshed with a subsequent
    `ray up`) this number of nodes will be started.
    
    It's a workaround for issues of scaling (see related issues) where it
    can take a long time (or forever in the case where the head node has
    `--num-cpus 0`) to scale up a cluster in response to increasing demand.


## Related issue number

Workaround for https://github.com/ray-project/ray/issues/3339 and https://github.com/ray-project/ray/issues/2106

python/ray/autoscaler/local/example-full.yaml

@@ -1,6 +1,7 @@
 cluster_name: default
 min_workers: 0
 max_workers: 0
+initial_workers: 0
 docker:
     image: ""
     container_name: ""