Fix bad usage of extend in list_image_import_opts
Fix bad usage of extend in list_image_import_opts

This change does not explode the list every time the
list_image_import_opts is called like 225a3396 did.

Change-Id: I865bface3b373e6f92bdbefa544d1fefa9da6990
Closes-Bug: #1748399

openstack/glance,openstack/glance,openstack/glance