rbd: only specify --format if not using the default
rbd: only specify --format if not using the default

This lets older versions that do not support --format still work with
format 1 images.

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