Update the report config file check to account for permissions

Right now the report only raises an error if the agent's config file
doesn't exist. It should also raise an error if the file isn't readable,
otherwise you get a surprising permissions error.

Author: mcasper

lib/puppet/reports/datadog_reports.rb

@@ -10,7 +10,7 @@
 Puppet::Reports.register_report(:datadog_reports) do
 
   configfile = "/etc/dd-agent/datadog.yaml"
-  raise(Puppet::ParseError, "Datadog report config file #{configfile} not readable") unless File.exist?(configfile)
+  raise(Puppet::ParseError, "Datadog report config file #{configfile} not readable") unless File.readable?(configfile)
   config = YAML.load_file(configfile)
   API_KEY = config[:datadog_api_key]