rbd.xfstests: default to 250mb instead of 100mb
rbd.xfstests: default to 250mb instead of 100mb

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