Review configuration file parsing logic

Luet support now these priorities on read configuration file:
- command line option (if available)
- $PWD/.luet.yaml
- $HOME/.luet.yaml
- /etc/luet/luet.yaml

pkg/helpers/sys.go

@@ -16,7 +16,9 @@
 package helpers
 
 import (
+	"os"
 	"os/exec"
+	"os/user"
 	"syscall"
 
 	"github.com/pkg/errors"
@@ -30,3 +32,17 @@ func Exec(cmd string, args []string, env []string) error {
 	}
 	return syscall.Exec(path, args, env)
 }
+
+func GetHomeDir() (ans string) {
+	// os/user doesn't work in from scratch environments
+	u, err := user.Current()
+	if err == nil {
+		ans = u.HomeDir
+	} else {
+		ans = ""
+	}
+	if os.Getenv("HOME") != "" {
+		ans = os.Getenv("HOME")
+	}
+	return ans
+}