When defining a ClusterIP Service, we can specify externalIP, and the
traffic policy of externalIP is subject to externalTrafficPolicy.
However, the policy can't be set when type is not NodePort or
LoadBalancer, and will default to Cluster when kube-proxy processes the
Service.
This commit updates the defaulting and validation of Service to allow
specifying ExternalTrafficPolicy for ClusterIP Services with
ExternalIPs.
Signed-off-by: Quan Tian <qtian@vmware.com>
