thrashosds: fix action selection
thrashosds: fix action selection

I'm not sure what the old code was trying to do, but I'm pretty sure it
wasn't doing it correctly.. a .1 chance_down was killing an OSD for me
virtually every time.

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