Automatic merge from submit-queue (batch tested with PRs 45573, 46354, 46376, 46162, 46366)
GCE - Retrieve subnetwork name/url from gce.conf
**What this PR does / why we need it**:
Features like ILB require specifying the subnetwork if the network is type manual.
**Notes:**
The network URL can be [constructed](68e7e18698/pkg/cloudprovider/providers/gce/gce.go (L211-L217)) by fetching instance metadata; however, the subnetwork is not provided through this feature. Users must specify the subnetwork name/url through the gce.conf.
Although multiple subnets can exist in the same region for a network, the cloud provider will only use one subnet url for creating LBs.
**Release note**:
```release-note
NONE
```
9c1480bb61