Worker API: Replace no update

[speedfl]

Hello,

Replace Worker Node supports replace with update or not, however in the sdk it is not possible to do a worker replace without update:

https://github.com/IBM-Cloud/bluemix-go/blob/master/api/container/containerv2/workers.go#L147

Two proposals:

1. No backward compatibility: we add a field update bool in the function signature
2. We create a function ReplaceNoUpdateWorkerNode

Let me think what you prefer and I can propose a PR

Have a good day