Username postfix for ldap_authenticators

ldap.rb has a username_prefix, but no username_postfix.

Scenario:
We use authentication based on the userprincipalname (an active directory attribute). Our userprincipalnames follows the syntax user@sub.domain.com. But we use only a partial for login: user@sub. In our large organisation there may be identical samaccountnames in different subdomains like smith@sub1 and smith@sub2.

If we can use a username_postfix = '.domain.com', all works fine: the user can use the short form and the authentication uses the full userprincipalname.