Honor envs properly for access and secret key. (#3703)

Also changes the behavior of `secretKeyHash` which is
not necessary to be sent over the network, each node
has its own secretKeyHash to validate.

Fixes #3696
Partial(fix) #3700 (More changes needed with some code cleanup)

cmd/config-v13_test.go

@@ -106,7 +106,7 @@ func TestServerConfig(t *testing.T) {
 	setGlobalConfigPath(rootPath)
 
 	// Initialize server config.
-	if _, err := initConfig(); err != nil {
+	if err := loadConfig(credential{}); err != nil {
 		t.Fatalf("Unable to initialize from updated config file %s", err)
 	}
 }