Update config.pp

Vagrant error:
Error: Illegal name. The given name _cluster_nodes does not conform to the naming rule \A((::)?[a-z0-9]w*)(::[a-z0-9]w*)*\z at /etc/puppet/modules/rabbitmq/manifests/config.pp:45:5
Error: Illegal name. The given name _cluster_nodes does not conform to the naming rule \A((::)?[a-z0-9]w*)(::[a-z0-9]w*)*\z at /etc/puppet/modules/rabbitmq/manifests/config.pp:47:5
Error: Found 2 errors. Giving up at /etc/puppet/modules/rabbitmq/manifests/config.pp:2 on node packer-virtualbox.vagrantup.com
Error: Found 2 errors. Giving up at /etc/puppet/modules/rabbitmq/manifests/config.pp:2 on node packer-virtualbox.vagrantup.com