thrasher: add option to mark OSDs down instead of out.
thrasher: add option to mark OSDs down instead of out.

Signed-off-by: Greg Farnum <0bcb4295d38ee745130ea5f2bb88ef4eef474e61@dreamhost.com>

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