Support "locked" filter when list servers

After the implementation of https://review.openstack.org/#/q/topic:bp/add-locking-information-in-server-get-response , we now have locked information in the response data of show method, but it is not good enough. As we may have a large number of locked instances, it will take forever to check the instances one by one, it will be really useful if we can list all the locked instances in one call.

This BP propose to add support for locked filter for List method.