Only define Cron['puppet_agent_once_at_boot'] if $run_method == 'cron'

On a new RHEL 6.4 system the following error occurs
"-":6: bad command
errors in crontab file, can't install.
Merge pull request #37 from tekenny/master

Only define Cron['puppet_agent_once_at_boot'] if $run_method == 'cron'