[#493] cmd/node: Implement a basic configuration component

Create `config` package nearby storage node application. Implement `Config`
as a wrapper over `viper.Viper` that provides the minimum functionality
required by the application.

The constructor allows you to read the config from the file. Methods are
provided for reading subsections and values from the config tree. Helper
functions are implemented to cast a value to native Go types.

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

cmd/neofs-node/config/calls_test.go

@@ -0,0 +1,29 @@
+package config_test
+
+import (
+	"testing"
+
+	"github.com/nspcc-dev/neofs-node/cmd/neofs-node/config"
+	"github.com/stretchr/testify/require"
+)
+
+func TestConfigCommon(t *testing.T) {
+	forEachFileType("test/config", func(c *config.Config) {
+		val := c.Value("value")
+		require.NotNil(t, val)
+
+		val = c.Value("non-existent value")
+		require.Nil(t, val)
+
+		sub := c.Sub("section")
+		require.NotNil(t, sub)
+
+		const nonExistentSub = "non-existent sub-section"
+
+		sub = c.Sub(nonExistentSub)
+		require.Nil(t, sub)
+
+		val = c.Sub(nonExistentSub).Value("value")
+		require.Nil(t, val)
+	})
+}