Add a 0-score for unknown score keys

Will print a warning message - so cron should email the owner of the job
when an unknown score key is encountered.

The example is blueprint db2-database which has "Not" as the priority

Change-Id: I4d43017be6a54bc3322d5aa754034a3d973d484d
Reviewed-on: https://review.openstack.org/36846
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Dan Prince <dprince@redhat.com>
Reviewed-by: Dan Prince <dprince@redhat.com>
Tested-by: Jenkins