Add a helper to fix neutron database after upgrade

The host used to register agents in the neutron database is not a FQDN
anymore, while it was in quantum. So we need to update the database
accordingly, otherwise we lose networking.

See https://bugs.launchpad.net/neutron/+bug/1236439 and
https://wiki.openstack.org/wiki/ReleaseNotes/Havana#Agents_May_Report_a_Different_Host_Name