Add rbd.mkfs method.
Add rbd.mkfs method.

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