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.
```
