rbd-mirror: Set the data pool correctly when creating images

rbd-mirror: Set the data pool correctly when creating images

Previously the data-pool option was ignored by rbd-mirror, so when using
erasure coding data for the image on the secondary site would end up in
the metadata pool.

Configure the data-pool using the data-pool's name from the primary site
when creating images on the secondary site.

Fixes: http://tracker.ceph.com/issues/20567
Signed-off-by: Adam Wolfe Gordon <awg@digitalocean.com>