mon: make 'osd blacklist rm ...' idempotent

Signed-off-by: Sage Weil <sage@inktank.com>
mon: make 'osd pool mksnap ...' idempotent

Signed-off-by: Sage Weil <sage@inktank.com>
mon: make 'osd pool rmsnap ...' idempotent

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #233 from ceph/wip-mon-idempotent

Wip mon idempotent

Reviewed-by: Dan Mick <dan.mick@inktank.com>