[#493] node/config: Change Sub implementation

In some cases viper doesn't interpret `section.value` as a subsection with
`section` name, but value is value still can be accessed through full
pathname.

Fix `Config.Sub` method implementation in order to always interpret
configuration like described above as a subsection. From now method never
returns nil, therefore an additional check has been removed from the `Value`
method.

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

cmd/neofs-node/config/config.go

@@ -13,7 +13,11 @@ import (
 // Sub-trees are named configuration sub-sections,
 // leaves are named configuration values.
 // Names are of string type.
-type Config viper.Viper
+type Config struct {
+	v *viper.Viper
+
+	path []string
+}
 
 const (
 	separator    = "."
@@ -49,5 +53,7 @@ func New(_ Prm, opts ...Option) *Config {
 		}
 	}
 
-	return (*Config)(v)
+	return &Config{
+		v: v,
+	}
 }