securityGroupOverride should be allowed for network load balancer

[sadi304]

/kind bug

1. What kops version are you running? The command kops version, will display
this information.

2. What Kubernetes version are you running? kubectl version will print the
version if a cluster is running or provide the Kubernetes version specified as
a kops flag.

3. What cloud provider are you using?

4. What commands did you run? What is the simplest way to reproduce this issue?

5. What happened after the commands executed?

6. What did you expect to happen?

7. Please provide your cluster manifest. Execute
kops get --name my.example.com -o yaml to display your cluster manifest.
You may want to remove your cluster name and other sensitive information.

8. Please run the commands with most verbose logging by adding the -v 10 flag.
Paste the logs into this report, or in a gist and provide the gist link here.

9. Anything else do we need to know?