s3tests: clone correct branch
s3tests: clone correct branch

Signed-off-by: Yehuda Sadeh <48638f9c2a62c4fd20be28d0bdad68f5cd041402@inktank.com>

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