osdmaptool: don't put progress on stdout

If one requests JSON output, the progress message pollutes the output;
don't do that, send it to stderr instead

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Merge pull request #824 from dmick/next

osdmaptool: don't put progress on stdout

Reviewed-by: Sage Weil <sage@inktank.com>