daklib/queue.py: add option to file prod as bug report
By using the "Bug" option after writing the prod message, the email will be submitted to the BTS with a severity of 'serious', while also CC'ing ftpmasters and the maintainer/changed-by/signer.
There is no need for a delay since the BTS allows filing bugs against non-existent packages. Once the package is introduced, britney will notice the bug and prevent migration to testing.
Please let me know if the changes here are sufficient or if you've got feedback.