Add support for alternative login IDs

[AFaust]

In some use cases customer want to support login via multiple user IDs, e.g. the user name, personal ID or email address. The LDAP authentication logic should support a configurable priority list of user attributes to check when resolving a user name to the distinguished name. The first attribute in the chain that succeeds to resolve a user wins. If multiple users are found in one iteration, the entire authentication attempt terminates due to ambiguity. The effective user ID after authentication should always be the configured user ID attribute.
Note: Some clients may not properly use the resolved user ID and instead continue using the requested user ID. Alfresco Mobile has been known to be affected by this. It is impossible to address such issues within the scope of this module.