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/backends.go

@@ -12,12 +12,8 @@ import (
 var (
 	// Backends is a global map of discovery backends indexed by their
 	// associated scheme.
-	backends map[string]Backend
-)
-
-func init() {
 	backends = make(map[string]Backend)
-}
+)
 
 // Register makes a discovery backend available by the provided scheme.
 // If Register is called twice with the same scheme an error is returned.
@@ -42,7 +38,7 @@ func parse(rawurl string) (string, string) {
 
 // ParseAdvertise parses the --cluster-advertise daemon config which accepts
 // <ip-address>:<port> or <interface-name>:<port>
-func ParseAdvertise(store, advertise string) (string, error) {
+func ParseAdvertise(advertise string) (string, error) {
 	var (
 		iface *net.Interface
 		addrs []net.Addr

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
 }