Handle newer btrfstools.
Handle newer btrfstools.

Newer btfs userland tools needs a -f like xfs instead of a prompt
for yes. Trusty needs this change.

Signed-off-by: Sandon Van Ness <a38d61b815b8053a15c463c80449b5c080334ec7@inktank.com>

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