embed: Make immutable defaults constant

This changes the two immutable defaults into constants which allows
packages embedding etcd to import them as const! If they are variables,
then you'll fail with "const initializer foo is not a constant".

embed/config.go

@@ -39,6 +39,9 @@ const (
 	DefaultMaxSnapshots = 5
 	DefaultMaxWALs      = 5
 
+	DefaultListenPeerURLs   = "http://localhost:2380"
+	DefaultListenClientURLs = "http://localhost:2379"
+
 	// maxElectionMs specifies the maximum value of election timeout.
 	// More details are listed in ../Documentation/tuning.md#time-parameters.
 	maxElectionMs = 50000
@@ -49,8 +52,6 @@ var (
 		"Choose one of \"initial-cluster\", \"discovery\" or \"discovery-srv\"")
 	ErrUnsetAdvertiseClientURLsFlag = fmt.Errorf("--advertise-client-urls is required when --listen-client-urls is set explicitly")
 
-	DefaultListenPeerURLs           = "http://localhost:2380"
-	DefaultListenClientURLs         = "http://localhost:2379"
 	DefaultInitialAdvertisePeerURLs = "http://localhost:2380"
 	DefaultAdvertiseClientURLs      = "http://localhost:2379"