[#493] node/config: Implement ENV variable key constructor

Add `internal.Env` function which converts path to config value to ENV
variable key.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

cmd/neofs-node/config/internal/env.go

@@ -1,8 +1,22 @@
 package internal
 
+import (
+	"strings"
+)
+
 // EnvPrefix is a prefix of ENV variables related
 // to storage node configuration.
 const EnvPrefix = "neofs"
 
 // EnvSeparator is a section separator in ENV variables.
 const EnvSeparator = "_"
+
+// Env returns ENV variable key for particular config parameter.
+func Env(path ...string) string {
+	return strings.ToUpper(
+		strings.Join(
+			append([]string{EnvPrefix}, path...),
+			EnvSeparator,
+		),
+	)
+}

cmd/neofs-node/config/internal/env_test.go

@@ -0,0 +1,15 @@
+package internal_test
+
+import (
+	"testing"
+
+	"github.com/nspcc-dev/neofs-node/cmd/neofs-node/config/internal"
+	"github.com/stretchr/testify/require"
+)
+
+func TestEnv(t *testing.T) {
+	require.Equal(t,
+		"NEOFS_SECTION_PARAMETER",
+		internal.Env("section", "parameter"),
+	)
+}