Enhance config.Namespace (#4339)

when unpacking into config.Namespace, namespaces can be disabled
via `enabled: false` now. For settings allowing exactly one configuration, this
can be used by users to enabled/disable namespace without having to comment
them out in the configuration file.

example usage:

```
type Config struct {
    Output common.ConfigNamespace
}
```

user can have at most one enabled:

```
output.namespace1:
  enabled: false

output.namespace2:
  enabled: true
```

libbeat/common/config.go

@@ -35,7 +35,8 @@ type Config ucfg.Config
 
 // ConfigNamespace storing at most one configuration section by name and sub-section.
 type ConfigNamespace struct {
-	C map[string]*Config `config:",inline"`
+	name   string
+	config *Config
 }
 
 type flagOverwrite struct {
@@ -327,33 +328,52 @@ func (f *flagOverwrite) Get() interface{} {
 	return f.value
 }
 
-// Validate checks at most one sub-namespace being set.
-func (ns *ConfigNamespace) Validate() error {
-	if len(ns.C) > 1 {
-		return errors.New("more then one namespace configured")
+// Unpack unpacks a configuration with at most one sub object. An sub object is
+// ignored if it is disabled by setting `enabled: false`. If the configuration
+// passed contains multiple active sub objects, Unpack will return an error.
+func (ns *ConfigNamespace) Unpack(cfg *Config) error {
+	fields := cfg.GetFields()
+	if len(fields) == 0 {
+		return nil
+	}
+
+	for _, name := range fields {
+		sub, err := cfg.Child(name, -1)
+		if err != nil {
+			// element is no configuration object -> continue so a namespace
+			// Config unpacked as a namespace can have other configuration
+			// values as well
+			continue
+		}
+
+		if !sub.Enabled() {
+			continue
+		}
+
+		if ns.name != "" {
+			return errors.New("more then one namespace configured")
+		}
+
+		ns.name = name
+		ns.config = sub
 	}
+
 	return nil
 }
 
 // Name returns the configuration sections it's name if a section has been set.
 func (ns *ConfigNamespace) Name() string {
-	for name := range ns.C {
-		return name
-	}
-	return ""
+	return ns.name
 }
 
 // Config return the sub-configuration section if a section has been set.
 func (ns *ConfigNamespace) Config() *Config {
-	for _, cfg := range ns.C {
-		return cfg
-	}
-	return nil
+	return ns.config
 }
 
 // IsSet returns true if a sub-configuration section has been set.
 func (ns *ConfigNamespace) IsSet() bool {
-	return len(ns.C) != 0
+	return ns.config != nil
 }
 
 func configDebugString(c *Config, filterPrivate bool) string {