Add option to set a service nodeport

This patch adds the option to set a nodeport when creating a NodePort
service. In case of a port allocation error due to a specified port
being out of the valid range, the error now includes the valid
range. If a `--node-port` value is not specified, it defaults to zero, in
which case the allocator will default to its current behavior of
assigning an available port.

This patch also adds a new helper function in `cmd/util/helpers.go` to
retrieve `Int32` cobra flags.

**Example**
```
$ kubectl create service nodeport mynodeport --tcp=8080:7777 --node-port=1
The Service "mynodeport" is invalid: spec.ports[0].nodePort: Invalid
value: 1: provided port is not in the valid range. Valid ports range
from 30000-32767

$ kubectl create service nodeport mynodeport --tcp=8080:7777 --node-port=30000
service "mynodeport" created

$ oc describe service mynodeport
Name:                   mynodeport
Namespace:              default
Labels:                 app=mynodeport
Selector:               app=mynodeport
Type:                   NodePort
IP:                     172.30.81.254
Port:                   8080-7777       8080/TCP
NodePort:               8080-7777       30000/TCP
Endpoints:              <none>
Session Affinity:       None
No events.
```

pkg/kubectl/service_basic.go

@@ -31,6 +31,7 @@ type ServiceCommonGeneratorV1 struct {
 	TCP       []string
 	Type      api.ServiceType
 	ClusterIP string
+	NodePort  int32
 }
 
 type ServiceClusterIPGeneratorV1 struct {
@@ -56,6 +57,7 @@ func (ServiceNodePortGeneratorV1) ParamNames() []GeneratorParam {
 	return []GeneratorParam{
 		{"name", true},
 		{"tcp", true},
+		{"nodeport", true},
 	}
 }
 func (ServiceLoadBalancerGeneratorV1) ParamNames() []GeneratorParam {
@@ -174,12 +176,14 @@ func (s ServiceCommonGeneratorV1) StructuredGenerate() (runtime.Object, error) {
 		if err != nil {
 			return nil, err
 		}
+
 		portName := strings.Replace(tcpString, ":", "-", -1)
 		ports = append(ports, api.ServicePort{
 			Name:       portName,
 			Port:       port,
 			TargetPort: targetPort,
 			Protocol:   api.Protocol("TCP"),
+			NodePort:   s.NodePort,
 		})
 	}