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.
I think that's probably an advantage of using isMemberOf/memberOf attributes, as the LDAP server tends to understand nesting.