Don't allow to define multiple health checks for AWS ELB

ELB doesn't support multiple health checks, terraform allowed
to define them, but it used the first one without any warning.
After this change it will fail.

builtin/providers/aws/resource_aws_elb.go

@@ -168,6 +168,13 @@ func resourceAwsElb() *schema.Resource {
 					},
 				},
 				Set: resourceAwsElbHealthCheckHash,
+				ValidateFunc: func(v interface{}, k string) (ws []string, errors []error) {
+					vs := v.([]interface{})
+					if len(vs) > 1 {
+						errors = append(errors, fmt.Errorf("ELB doesn't support multiple health checks"))
+					}
+					return
+				},
 			},
 
 			"dns_name": &schema.Schema{