allow s3tests.create_users defaults be overridden
allow s3tests.create_users defaults be overridden

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