AWS availability zone should *not* be hard coded

[maximilien]

Currently it is hardcoded in the controller.go

func CreateController(instanceMap map[string]*model.ServiceInstance, bindingMap map[string]*model.ServiceBinding) *Controller {
    return &Controller{
        instanceMap:   instanceMap,
        bindingMap:    bindingMap,
        serviceClient: ac.NewClient("us-west-1"),
    }
}