rbd-mirror: Improve data pool selection when creating images

rbd-mirror: Improve data pool selection when creating images

Previously we used the source image's data pool name
unconditionally. There were two problems with that:

1. If a pool with the same name didn't exist locally, creation of the
   local image would fail.
2. If the local pool had a default data pool configured it would be
   ignored.

Change local image creation so it uses the default pool if configured,
and uses the remote pool name only if a pool with that name exists
locally. If neither of those is true, leave the data pool unset.

Signed-off-by: Adam Wolfe Gordon <awg@digitalocean.com>