Automatic merge from submit-queue (batch tested with PRs 50769, 50739)

Support autoprobing subnet-id for openstack cloud provider

Currently if user doesn't specify subnet-id or specify a unsafe
subnet-id, openstack cloud provider can't create a correct LoadBalancer
service.
Actually we can get it automatically. This patch do a improvement.
This is a part of #50726

**Special notes for your reviewer**:
/assign @dims 
/assign @anguslees 

**Release note**:
```release-note
NONE
```