test: use ipv6_only IPv6 addresses in custom cluster integration tests. (#6260)
test: use ipv6_only IPv6 addresses in custom cluster integration tests. (#6260)

Needed for IPv6-only environments such as Google.

Risk level: Low
Testing: custom_cluster_integration_test in IPv6 only environment.

Signed-off-by: Harvey Tuch <c185081708e02336640916420d0c772668570de5@google.com>
envoyproxy/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,lizan/envoy,lizan/envoy,eklitzke/envoy,istio/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,lizan/envoy,dnoe/envoy,dnoe/envoy,jrajahalme/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,dnoe/envoy,envoyproxy/envoy,istio/envoy,dnoe/envoy,istio/envoy,envoyproxy/envoy,eklitzke/envoy,jrajahalme/envoy,lyft/envoy,eklitzke/envoy,jrajahalme/envoy,eklitzke/envoy,istio/envoy,dnoe/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,istio/envoy,lizan/envoy,lizan/envoy,envoyproxy/envoy,lyft/envoy,lyft/envoy,jrajahalme/envoy,istio/envoy,eklitzke/envoy,envoyproxy/envoy-wasm,lizan/envoy,lyft/envoy,istio/envoy,envoyproxy/envoy-wasm,eklitzke/envoy,envoyproxy/envoy,lyft/envoy,jrajahalme/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,jrajahalme/envoy