This replaces the 'glance task-list' command.
$ openstack image task list
We also indicate that the 'image task create' command will never be
implemented. This is an admin-only API that isn't really intended to be
used by humans thus it does not need an OSC command implementation.
Change-Id: Id8a943a5443782fc70c0fbf3639f5aa17b9d30af