fix error on teardown failing to unmount /mnt
fix error on teardown failing to unmount /mnt

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