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>

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