Make floatingip-ip-delete atomic with neutron

The infra guys were noticing an issue where they were leaking floating ip
addresses. One of the reasons this would occur for them is they called
nova floating-ip-delete which first disassocates the floating-ip in neutron
and then deletes it. Because nova-api makes two calls to neutron if the
first one succeeds and the second fails it results in the instance no longer
being associated with the floatingip. They have retry logic but they base it on
the instance and when they go to retry cleaning up the instance the floatingip
is no longer on the instance so they never delete it.

This patch fixes this issue by creating a new method
disassocate_and_delete_floating_ip which contains the different logic
needed to atomically do this for neutron rather than putting the logic
in the api layer to do something differently if one is using neutron.

This patch also adds additional code coverage for nova-network in the
delete floatingip case.

Closes-bug: 1356157
Change-Id: I53b0c9d949404288e8687b304361a74b53d69ef3
Merge "Make floatingip-ip-delete atomic with neutron"