mon: make 'osd crush rm|unlink ...' idempotent

Return 0 if the item is not there, not -ENOENT.

Fixes: #4624
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #193 from ceph/wip-mon-crush

mon: make 'osd crush rm|unlink ...' idempotent

Reviewed-by: Dan Mick dan.mick@inktank.com