Update manifests/repo/rhel.pp

Avoids that RPM GPG-Key import is executed at every Puppet run.

Rabbitmq::Repo::Rhel/Exec[rpm --import http://www.rabbitmq.com/rabbitmq-signing-key-public.asc]/returns: executed successfully
Merge pull request #39 from gvarisco/master

Fixes RPM GPG-key import exec at every puppet run