[#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/cast.go

@@ -0,0 +1,30 @@
+package config
+
+import (
+	"github.com/spf13/cast"
+)
+
+func panicOnErr(err error) {
+	if err != nil {
+		panic(err)
+	}
+}
+
+// StringSlice reads configuration value
+// from c by name and casts it to []string.
+//
+// Panics if value can not be casted.
+func StringSlice(c *Config, name string) []string {
+	x, err := cast.ToStringSliceE(c.Value(name))
+	panicOnErr(err)
+
+	return x
+}
+
+// StringSliceSafe reads configuration value
+// from c by name and casts it to []string.
+//
+// Returns nil if value can not be casted.
+func StringSliceSafe(c *Config, name string) []string {
+	return cast.ToStringSlice(c.Value(name))
+}