[#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_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"),
+	)
+}