rbd_fsx: expose krbd and related fsx options

Expose

-K (enable krbd mode) through 'krbd',
-U (disable randomized striping) through 'randomized_striping',
-H (disable discard ops) through 'punch_holes',

-r readbdy (read alignment) through 'readbdy',
-w writebdy (write alignment) through 'writebdy',
-h holebdy (discard alignment) through 'holebdy'.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>