Don't fetch owner info from job_info when --owner given

Commit 293c2bd49c0d ("Be more informative if owner info is missing")
introduced --owner but it is still pulled from job_info no matter what.
Fix it.

Signed-off-by: Ilya Dryomov <idryomov@redhat.com>
No need to invoke teuthology-nuke if there is nothing to nuke

Slipped in with 26803da18d1b ("Rewrite portions of teuthology.kill").

Signed-off-by: Ilya Dryomov <idryomov@redhat.com>
Merge pull request #377 from ceph/wip-kill-fixes

Two minor teuthology-kill fixes