[GKE] allow setting svc spec.loadBalancerIP for GKE reserved/static IPs

[drnic]

[GKE] To use a reserved/static IP for LoadBalancer services you need to set spec.loadBalancerIP, rather than spec.externalIPs. This PR allows the following values to be set for GKE:

services:
  router:
    loadBalancerIP: 34.82.240.78
    externalIPs: []
  ssh-proxy:
    loadBalancerIP: 34.82.240.79
    externalIPs: []
  tcp-router:
    loadBalancerIP: 34.82.240.80
    externalIPs: []

How Has This Been Tested?

• against cf-operator v1.0.0
• on GKE
• only for services.router.loadBalancerIP to confirm my kube service switched to use my reserved IP

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code has security implications.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[fargozhu]

fixes #291

[f0rmiga]

@drnic Thanks for fixing this one!
@fargozhu We need a doc update for GKE.

[fargozhu]

after deploying kubecf with PR merged and set the values.yaml to support the described approach, I can see the endpoints created with the specified IPs but the k8s cluster EXTERNAL-IP column (router, ssh and tcp) are in pending state.

kubecf        kubecf-router-public           LoadBalancer   10.88.10.228   <pending>     80:32697/TCP,443:30861/TCP                                                                                                                        129m

❯ k describe svc kubecf-router-public -n kubecf
Name:                     kubecf-router-public
Namespace:                kubecf
Labels:                   app.kubernetes.io/component=router
                          app.kubernetes.io/instance=kubecf
                          app.kubernetes.io/managed-by=Helm
                          app.kubernetes.io/name=kubecf
                          app.kubernetes.io/version=0.1.0
                          helm.sh/chart=kubecf-0.1.0
Annotations:              <none>
Selector:                 app.kubernetes.io/component=router,app.kubernetes.io/instance=kubecf,app.kubernetes.io/version=0.1.0
Type:                     LoadBalancer
IP:                       10.88.10.228
IP:                       35.214.153.155
Port:                     http  80/TCP
TargetPort:               8000/TCP
NodePort:                 http  32697/TCP
Endpoints:                10.24.1.11:8000
Port:                     https  443/TCP
TargetPort:               443/TCP
NodePort:                 https  30861/TCP
Endpoints:                10.24.1.11:443
Session Affinity:         None
External Traffic Policy:  Cluster
Events:
  Type    Reason                Age                    From                Message
  ----    ------                ----                   ----                -------
  Normal  EnsuringLoadBalancer  2m18s (x33 over 138m)  service-controller  Ensuring load balancer