Fix swift-recon-cron lock errors

Depending on timing, and if you're running the chef client install pre or post
ring creation, the swift-recon-cron job might run, fail on no drives, die with
an error and leave the lock_dir in place. On the second run, cron gets angry
and the lock_dir must be removed before this task will ever run properly even
after ring/drive setup.

Only run the cron job if some form of /etc/swift/*.ring.gz exists.

Fixes rcbops-cookbooks/swift-private-cloud#235