add backfill task
add backfill task

This does a basic test of backfill functionality, including a divergent
log on a backfill target (#1983).

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