make report times a little more user friendly

report times as 1h 22m 08s

only report hours if the job runs longer than an hour

only report minutes if the job runs longer than a minute

don't zero pad whatever the leading time unit is, making it a
little easier to scan and see differences.

Change-Id: Ibb58be233fdef1bbdf4e90a83731d43eb0be47f1
Reviewed-on: https://review.openstack.org/28143
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: matthew-wagoner <matthew.wagoner@hp.com>
Reviewed-by: Antoine Musso <hashar@free.fr>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins