teuthology/suite/test: clarify variable name
teuthology/suite/test: clarify variable name

"den" for denominator is not that intuitive.

Signed-off-by: Patrick Donnelly <d35c2028e065409a3eff47e1b4cba347a9379b4b@redhat.com>

ceph/teuthology,ceph/teuthology,ktdreyer/teuthology,ktdreyer/teuthology