rbd: allow specifying all clients
rbd: allow specifying all clients

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