document 'command' requirements on admin_socket method

Signed-off-by: Greg Farnum <greg@inktank.com>
mds_scrub_checks: Run scrub and flush commands against the MDS.

We mostly do a variety of successful ones, but we also corrupt the store
using the rados tool and make sure we get the expected error codes. Includes
a yaml fragment so the task gets run as part of the fs/basic suite.

Signed-off-by: Greg Farnum <greg@inktank.com>
Merge pull request #221 from ceph/wip-forward-scrub

Wip forward scrub

Reviewed-by: John Spray <john.spray@redhat.com>