No need to create worker statefulsets when size=1

[kerthcet]

What would you like to be added:

Once we set the size = 1, if we have 10 replicas, we'll have 10 0-replica statefulsets, there's no need to create these statefulsets.

Why is this needed:

When using lws as single-host workload(size=1), this leads to confusion and additional computations.

cc @ahg-g @liurupeng

Completion requirements:

This enhancement requires the following artifacts:

• Design doc
• API change
• Docs update

The artifacts should be linked in subsequent comments.

[ahg-g]

sounds good to me