rbd: make --pool/--image args easier to understand for import

rbd: make --pool/--image args easier to understand for import

There's no need to set the default pool in set_pool_image_name - this
is done later, in a way that doesn't ignore --pool if --dest-pool
is not specified.

This means --pool and --image can be used with import, just like
the rest of the commands. Without this change, --dest and --dest-pool
had to be used, and --pool would be silently ignored for rbd import.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>