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 could be simplified to:
```ruby
def user_exists?(uid)
  @member_service.find_user(uid)
  true
rescue self.class::MemberService::UIDNotFoundException
  false
end
```
please look also for other usages.