rbd_xfstests: re-enable test 049

Test 049 is now passing, so add it back to the set that runs
nightly.  It only takes 10-15 seconds to run, so it shouldn't
adversely affect the balance of tests run across the three clients.

This resolves:
    http://tracker.ceph.com/issues/4244

Signed-off-by: Alex Elder <elder@inktank.com>