Add status.podIP as a valid downward API target

Getting the public IP a container is supposed to use is O(hard),
and usually involves ugly gyrations in python or with interfaces.
Using the downward API means that the IP Kube is announcing to
other endpoints is also visible inside the container for pods to
identify themselves.

pkg/api/validation/validation_test.go

@@ -667,7 +667,7 @@ func TestValidateEnv(t *testing.T) {
 					},
 				},
 			}},
-			expectedError: "[0].valueFrom.fieldRef.fieldPath: unsupported value 'status.phase', Details: supported values: metadata.name, metadata.namespace",
+			expectedError: "[0].valueFrom.fieldRef.fieldPath: unsupported value 'status.phase', Details: supported values: metadata.name, metadata.namespace, status.podIP",
 		},
 	}
 	for _, tc := range errorCases {