Add option to report elapsed build times.

The new format will be:
  "http://logs.example.com/6/1/gate/project2-merge/2 : SUCCESS in 00:00:00"

And the option is enabled by default.

Change-Id: Ib50c4948ea0a7b552d46a0e72ecb6c1a9609a771
Reviewed-on: https://review.openstack.org/27570
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins