Fix finding IPv4 subnets assigned to KinD clusters

[jewertow]

Currently, we do not support east-west gateways with IPv6 address, and the KinD provisioner script was not checking which subnet it gets from the container config, so the CI checks fail due to wrong IP address pool:

++ docker inspect kind
++ jq '.[0].IPAM.Config[0].Subnet' -r
+ docker_kind_subnet=fc00:f853:ccd:e793::/64
++ python3 /work/test/scripts/find_smaller_subnets.py --network fc00:f853:ccd:e793::/64 --region west
+ cidr=fc00:f853:ccd:e793:4000::/66
+ echo '
apiVersion: metallb.io/v1beta1
kind: IPAddressPool
metadata:
  name: default-pool
  namespace: metallb-system
spec:
  addresses:
  - fc00:f853:ccd:e793:4000::/66
  avoidBuggyIPs: true
---
apiVersion: metallb.io/v1beta1
kind: L2Advertisement
metadata:
  name: default-l2
  namespace: metallb-system
spec:
  ipAddressPools:
  - default-pool
'
...
2024-10-04T05:16:28.358314Z	info	tf	=== DONE: Building clusters ===
2024-10-04T05:16:28.358328Z	info	tf	=== BEGIN: Setup: 'test_e2e' ===
2024-10-04T05:16:34.908807Z	error	tf	Test setup error: could not get IPs from remote federation-controller: no load balancer IP found for service istio-eastwestgateway/istio-system in cluster cluster-1
2024-10-04T05:16:34.908912Z	info	tf	=== FAILED: Setup: 'test_e2e' (could not get IPs from remote federation-controller: no load balancer IP found for service istio-eastwestgateway/istio-system in cluster cluster-1) ===
2024-10-04T05:16:34.908938Z	error	tf	Exiting due to setup failure: could not get IPs from remote federation-controller: no load balancer IP found for service istio-eastwestgateway/istio-system in cluster cluster-1
FAIL	github.com/jewertow/federation/test/e2e	6.626s