juju-kill: don't automatically reap()

Behavior change request from @mikemccracke: juju-kill no longer reap()s
everything, it just destroys everything in the "tree" that you mention. You can
still force a reap() via --reap.
units don't always have machines
Merge pull request #15 from tych0/no-default-reap

juju-kill: don't automatically reap()