rbd: make default image 10G instead of 1G
rbd: make default image 10G instead of 1G

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