Optimize check for RHEL GPG key

Reduces the time to check for presence of
gpg pubkey for RabbitMQ from 1.6s to 0.1s.

manifests/repo/rhel.pp

@@ -6,7 +6,7 @@
 
   exec { "rpm --import ${package_gpg_key}":
     path   => ['/bin','/usr/bin','/sbin','/usr/sbin'],
-    onlyif => 'test `rpm -qa | grep gpg-pubkey-056e8e56-468e43f2 | wc -l` -eq 0',
+    unless => 'rpm -q gpg-pubkey-056e8e56-468e43f2 2>/dev/null',
   }
 
 }