scripts/suite.py: remove default -k (kernel) value

Note: will require current cron jobs to change to request -k testing
explicitly.  Removes current special meaning of '-k -'.

Fixes: #10304
Signed-off-by: Dan Mick <dan.mick@redhat.com>
Merge pull request #394 from ceph/wip-default-kernel

scripts/suite.py: change default kernel from 'testing' to '-'