juju fails to delete openstack security groups when a controller is killed

[Jason Hobbs]

When I kill a controller that's been bootstrapped in openstack, I get warnings that a security group can't be deleted, and it indeed isn't deleted:

+ juju kill-controller --yes vpil-openstack-lxd-controller
Destroying controller "vpil-openstack-lxd-controller"
Waiting for resources to be reclaimed
Waiting on 1 model
Waiting on 1 model
All hosted models reclaimed, cleaning up controller machines
WARNING cannot delete security group "juju-b52f09c2-bd28-441c-8d49-0bfa8db94027-b52f09c2-bd28-441c-8d49-0bfa8db94027-0". Used by another model?

This is with 2.0-beta10-0ubuntu1~16.04.1~juju1