[#493] node/config: Implement logger section

Create `logger` sub-package of `config` package. Implement `LoggerSection`
type of logger sub-section. Add `Level` method to read logger level config
value. Default level is `info`.

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

cmd/neofs-node/config/logger/config.go

@@ -0,0 +1,33 @@
+package loggerconfig
+
+import (
+	"github.com/nspcc-dev/neofs-node/cmd/neofs-node/config"
+)
+
+// LoggerSection represents config section
+// for logging component.
+type LoggerSection config.Config
+
+// config defaults
+const (
+	// LevelDefault is a default logger level
+	LevelDefault = "info"
+)
+
+// Init initializes LoggerSection from
+// "logger" subsection of config.
+func Init(root *config.Config) *LoggerSection {
+	return (*LoggerSection)(root.Sub("logger"))
+}
+
+// Level returns configuration value with name "level".
+//
+// Returns LevelDefault if value is not a non-empty string.
+func (x *LoggerSection) Level() string {
+	v := config.StringSafe((*config.Config)(x), "level")
+	if v != "" {
+		return v
+	}
+
+	return LevelDefault
+}