radosbench: cleanup will probably take longer than writing the objects

With thrashing, cleanup will require recovering the degraded objects
first, while the initial write out doesn't since they they are new
objects.

Signed-off-by: Samuel Just <sam.just@inktank.com>
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 <sandon@inktank.com>
Merge remote-tracking branch 'gh/firefly'