Allow users to configure standard load balancer for k8s

[feiskyer]

Kubernetes v1.11 adds standard load balancer support. It adds two configs to /etc/kubernetes/azure.json:

{
"loadBalancerSku": "standard",
"excludeMasterFromStandardLB": true,
}

If loadBalancerSku is not set, it will default to basic (which is same as today).

If excludeMasterFromStandardLB is not set, it will default to true, which means master nodes are excluded to the backend of standard LB.

acs-engine should support customize those two configs.