Speed up teuthology-kill.

Read less files, don't look at mtime of files, log.info() the job
being read so you know something is happening so there is never a
situation where 'assembling build information' just sits there for
minutes with no output. If that happens now there is a seirous
problem.

Tested speed increase was 2-3x.

Signed-off-by: Sandon Van Ness <sandon@inktank.com>