Move foreman LDAP functionality to ldap_fluff

Most logic comes from here: https://github.com/theforeman/foreman/blob/develop/app/models/auth_sources/auth_source_ldap.rb 

and https://github.com/theforeman/foreman/pull/529 , which this PR is meant to facilitate.
This doesn't handle memberUid correctly (as used from users_for_gid), as memberUid may be just the username or UID, while this only returns the users if they're in the form "attribute=uid".  It currently returns an empty array.