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
Merge pull request #236 from claco/issue-235

Merged automatically by jenkins after successful build test