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>
Well... This can still take a significant amount of time. A single job should not take THAT long to read. The problem is people have ran teuthology-kill and then control-c it because they think its hanging when in reality it might just be a big test with *a lot of jobs*. By adding some more output (by default) we actually know teuthology-kill is doing something instead of thinking its hanging.