backfill: use 'rbd' pool instead of 'data'
backfill: use 'rbd' pool instead of 'data'

(data has a replay interval, which makes writes take longer to resume
after repeering)

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