Decouple authentication backend creation from Icinga\Authentication\Manager

Add authentication backend type msldap with default values for user_class
and user_name_attribute. Backend type ldap now logs an error when user_class
and user_name_attribute ist not configured. Rename membership.ini to
memberships.ini since all our INI configuration files are in the plurar
where it makes sense. The AuthenticationController now handles
authentication

refs #5685
refs #5638
fixes #5218