krbd: add librbd_fsx -K job

Our fsx fork has recently gained a support for the krbd mode - add it
to the test suite.  Disable discard ops and randomized striping, both
of which aren't ready yet.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Merge pull request #38 from ceph/wip-krbd-fsx

krbd: add librbd_fsx -K job

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>