Automatic merge from submit-queue (batch tested with PRs 54230, 58100, 57861, 54752). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

GCE: Use existing subnetwork of ILB forwarding rule

Fixes #57860

**Test Cases**:
Clusters using auto network with existence of a manual subnet in same region.
- [x] Upgrade 1.7 cluster with existing ILBs to latest. Confirm existing ILBs still are synced.
  Version 1.7 does not attempt to fill in the subnetwork, so the forwarding rule was created with the correct subnetwork. 
- [x] Upgrade 1.8 cluster with existing ILBs to latest. Confirm existing ILBs (using wrong subnet) still are synced. 
- [x]  Latest version creates ILBs using the correct subnet.


Clusters with manual subnets have always and will continue to use the subnet specified in gce.conf.
- [x] Upgrade 1.8 cluster with existing ILBs to latest. Confirm existing ILBs (using manual subnet) still are synced. 


Clusters with legacy networks have always and will continue to use an empty subnet. 
- [x] Upgrade 1.8 cluster with existing ILBs to latest. Confirm existing ILBs (using legacy network) still are synced. 


**Release note**:
```release-note
GCE: Allows existing internal load balancers to continue using an outdated subnetwork 
```