Delete docs-draft files after 30 days.

They can be recreated if needed so let's just keep them around long
enough to be useful in code reviews.

Change-Id: I2cac9a75190b1db23df67568b1973b25c4179590
Reviewed-on: https://review.openstack.org/32039
Reviewed-by: Anne Gentle <anne@openstack.org>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins