(#1092) add an external discovery method

[ripienaar]

Signed-off-by: R.I.Pienaar rip@devco.net

[ripienaar]

this is just to test the arg parsing works, and probably not even a good test, wouldnt be needed normally

[ripienaar]

Using this with the go-external helper making a new discovery method is basically:

func main() {
	jsonDiscovery := discovery.NewDiscovery(func(ctx context.Context, timeout time.Duration, collective string, filter discovery.Filter) ([]string, error) {
		f, _ := ioutil.ReadFile("/etc/choria/nodes.json")
		nodes := []string{}
		err := json.Unmarshal(f, &nodes)
		
		return nodes, err
	})
	
	jsonDiscovery.ProcessRequest()
}