Allow to set daemon and server configurations in a file.

Read configuration after flags making this the priority:

1- Apply configuration from file.
2- Apply configuration from flags.

Reload configuration when a signal is received, USR2 in Linux:

- Reload router if the debug configuration changes.
- Reload daemon labels.
- Reload cluster discovery.

Signed-off-by: David Calavera <david.calavera@gmail.com>

discovery/memory/memory.go

@@ -25,6 +25,7 @@ func Init() {
 // Initialize sets the heartbeat for the memory backend.
 func (s *Discovery) Initialize(_ string, heartbeat time.Duration, _ time.Duration, _ map[string]string) error {
 	s.heartbeat = heartbeat
+	s.values = make([]string, 0)
 	return nil
 }