Properly clean up router VMs when they're removed from Neutron.

This is something I unintentionally broke awhile back (there wasn't a test for it):

https://github.com/dreamhost/akanda-rug/commit/272a42ada879bfed2b9910ef9aef7663b7aba76f
https://github.com/dreamhost/akanda-rug/commit/272a42ada879bfed2b9910ef9aef7663b7aba76f#diff-671f5dab54e3322917499496cdb8cbeeR69