Support attach/detach interface under cells v1

The code is up, GoDaddy has been running with this in production. It's not a very complicated change and I think we can enable the tempest testing on it in tree to verify it works, so I'd be in favor of approving this without a spec. -- mriedem 20151014

In discussing this in the nova IRC channel, os-attach-interfaces is only implemented for neutron (in nova), nova-net doesn't support that API. The gate-tempest-dsvm-cells job runs with nova-net, so we wouldn't actually have test coverage there for the change, we'd need a cells + neutron job (probably in the experimental queue or non-voting check queue) in order to validate this in the community CI system. -- mriedem 20151014

https://review.openstack.org/#/c/215459/

I unapproved this since we didn't have consensus in the nova channel, and I think we agreed that we wanted a cells + neutron job to verify this, which we don't have today. -- mriedem 20151014

Seems like we need a little spec adding to get this approved. --johnthetubaguy 20151104