radosbench: fix missing format value
radosbench: fix missing format value

tdir is substituted in at the end.  There is probably a better way to do
this.

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