RadosModel: allow --no-omap to be specified seperately from --ec-pool

Fixes: #7871
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1537 from ceph/wip-7871

RadosModel: allow --no-omap to be specified seperately from --ec-pool

Reviewed-by: Sage Weil <sage@inktank.com>