LDAP-Auth, additional option to login user when no group matched during group based search

For some LDAP architecture, there is no such thing as a "default group", only specific users are parts of specific group. In this case the plugin can only be use to either authentify anyone as default User role, or only authentificate specific group. Since "LDAP users without any valid LDAP group will be refused login".

This small PR add a new checkbox to let users choose to either refused login or login as default User role if no valid LDAP group has been found.