rados.py: add option to do many short runs
rados.py: add option to do many short runs

Signed-off-by: Samuel Just <8c40222f34b9721b5d5709d4ed45337913ea1c3e@inktank.com>
Reviewed-by: Josh Durgin <51652c783f70f5b272ebd02f1fd20d3af7a2449f@inktank.com>

SUSE/teuthology,robbat2/teuthology,michaelsevilla/teuthology,caibo2014/teuthology,robbat2/teuthology,SUSE/teuthology,dmick/teuthology,tchaikov/teuthology,ceph/teuthology,tchaikov/teuthology,dmick/teuthology,ktdreyer/teuthology,ktdreyer/teuthology,t-miyamae/teuthology,michaelsevilla/teuthology,SUSE/teuthology,ivotron/teuthology,caibo2014/teuthology,yghannam/teuthology,ivotron/teuthology,ceph/teuthology,t-miyamae/teuthology,zhouyuan/teuthology,dreamhost/teuthology,dreamhost/teuthology,dmick/teuthology,yghannam/teuthology,zhouyuan/teuthology