marginal: add multimds collection

Lots of stuff here, and several things fail with the export thrashing on,
but usually the others pass.

Signed-off-by: Sage Weil <sage@inktank.com>
marginal: remove partial collection thrash

Incomplete and broken.  The ffsb test is part of the kernel suite now.

Signed-off-by: Sage Weil <sage@inktank.com>
marginal: remove incomplete librbd collection

Move disabled qemu + iozone task into rbd suite.

Signed-off-by: Sage Weil <sage@inktank.com>