etcdmain: improve readability

Improve readability of ETCD_CONFIG_FILE env variable parsing part
by adding comments and using flags.FlagToEnv function.

Signed-off-by: Andrey Abramov <st5pub@yandex.ru>

etcdmain/config.go

@@ -284,8 +284,11 @@ func (cfg *config) parse(arguments []string) error {
 
 	var err error
 
+	// This env variable must be parsed separately
+	// because we need to determine whether to use or
+	// ignore the env variables based on if the config file is set.
 	if cfg.configFile == "" {
-		cfg.configFile = os.Getenv("ETCD_CONFIG_FILE")
+		cfg.configFile = os.Getenv(flags.FlagToEnv("ETCD", "config-file"))
 	}
 
 	if cfg.configFile != "" {