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/registry/core/service/portallocator/allocator.go

@@ -82,7 +82,10 @@ func (r *PortAllocator) Free() int {
 func (r *PortAllocator) Allocate(port int) error {
 	ok, offset := r.contains(port)
 	if !ok {
-		return ErrNotInRange
+		// include valid port range in error
+		validPorts := r.portRange.String()
+		msg := fmt.Sprintf("Valid ports range is %s", validPorts)
+		return fmt.Errorf("%v. %s", ErrNotInRange, msg)
 	}
 
 	allocated, err := r.alloc.Allocate(offset)