fix golint issues in core/discov (#479)

core/discov/config.go

@@ -2,11 +2,13 @@ package discov
 
 import "errors"
 
+// EtcdConf is the config item with the given key on etcd.
 type EtcdConf struct {
 	Hosts []string
 	Key   string
 }
 
+// Validate validates c.
 func (c EtcdConf) Validate() error {
 	if len(c.Hosts) == 0 {
 		return errors.New("empty etcd hosts")