Enable undef to service_ensure for better cluster management

In cluster environments, where haproxy is managed by corosync, is very needed that module doesn't enforce service by ensure. Added 'undef' state, that enables this and keeps the service notifying when config files changed. I would like to do this better, but seems there is no nice out of box solution, until service provider will accept 'undef' string such discussed at https://projects.puppetlabs.com/issues/22587#note-2 .