Audit backend interface for cross platform compatibility

[bprashanth]

I haven't had the time to prototype enough loadbalancer backends (eg: envoy, h2o, cloudprovider) with the generic_controller's interface. Currently it's pretty minimalistic, and captured in this commit: f908fb9

Tl;dr is an interface:

type Controller interface {
	Test(file string) *exec.Cmd
	OnUpdate(*api.ConfigMap, Configuration) ([]byte, error)
	BackendDefaults() defaults.Backend
	Info() *BackendInfo
}

and the Configuration type

type Configuration struct {
	HealthzURL         string
	Backends           []*Backend
	Servers            []*Server
	TCPEndpoints       []*Location
	UPDEndpoints       []*Location
	PassthroughBackend []*SSLPassthroughBackend
}

We need to attack this from 2 angles:

1. Are the methods in this interface enough to give the backends the flexibility they need?
2. Is the information passed through OnUpdate adequate

Right now, every onUpdate effectively gets:

Configuration
  Server 
    virtual server -> location -> Backends ([]endpoitns + ssl) -> Endpoints (address:port, health check info)
                ...more locations 
    ...more virtual servers
  ...more Servers

[aledbf]

@bprashanth in the location you also get information about the annotations (for instance ExternalAuth)

[aledbf]

Closing. Interface already improved and use by third party controllers