Handle the case where no key is installed

If there is currently no key present in the RPM keyring rpm -q returns
with a non-zero exitcode. This case is now handled properly so no
error message is shown when running puppet.

lib/puppet/provider/rpmkey/rpm.rb

@@ -4,7 +4,14 @@ Puppet::Type.type(:rpmkey).provide(:rpm) do
 
   def self.instances
     keys = []
-    rpm('-q','gpg-pubkey').each_line do |line|
+
+    begin
+      rpm_query = rpm('-q','gpg-pubkey')
+    rescue Puppet::ExecutionFailure
+      return []
+    end
+
+    rpm_query.each_line do |line|
       if match = /^gpg-pubkey-([0-9a-f]*)-[0-9a-f]*/.match(line)
         keys << new(:name => match.captures[0].upcase, :ensure => :present)
       else

spec/unit/provider/rpmkey/rpm_spec.rb

@@ -21,6 +21,11 @@ describe Puppet::Type.type(:rpmkey).provider(:rpm) do
         '23A254D4'
       ]
     end
+
+    it "should return an empty list if no key is installed" do
+      described_class.expects(:rpm).with('-q', 'gpg-pubkey').raises Puppet::ExecutionFailure, 'package gpg-pubkey is not installed'
+      described_class.instances.should be_empty
+    end
   end
 
   describe "#exists?" do