Add Host field to TCPSocketAction

Currently, TCPSocketAction always uses Pod's IP in connection. But when a
pod uses the host network, sometimes firewall rules may prevent kubelet
from connecting through the Pod's IP. This PR introduces the 'Host' field
for TCPSocketAction, and if it is set to non-empty string, the probe will
be performed on the configured host rather than the Pod's IP. This gives
users an opportunity to explicitly specify 'localhost' as the target for
the above situations.

pkg/api/types.go

@@ -1391,6 +1391,9 @@ type TCPSocketAction struct {
 	// Required: Port to connect to.
 	// +optional
 	Port intstr.IntOrString
+	// Optional: Host name to connect to, defaults to the pod IP.
+	// +optional
+	Host string
 }
 
 // ExecAction describes a "run in container" action.