Notify nova when ports are ready

Notify nova when ports are ready

The following patch adds a callback from neutron to nova that notifies nova
when a port for an instance is *ready to be used*. After nova receives this
event it will then start the instance in a hope that when it comes up
its networking should be in working order.

NOTE: *ready to be used* currently means that a plugin changes the status
in the db associated with a port from:
NO_VALUE/PORT_STATUS_DOWN/PORT_STATUS_ACTIVE to ACTIVE/ERROR.

Neutron will then signal nova: network_vif_plugged:<status> where status
will either be 'completed' or 'failed' given the neutron port status.

Neutron also notifies nova when a port goes from status:
PORT_STATUS_ACTIVE to PORT_STATUS_DOWN and sends nova a network_vif_unplugged
event.

Currently this patch breaks multiregion support (i.e previously you could
back multiple nova regions by one neutron server) though now since neutron
needs to notify nova we'll need to add a way to determine which region a
given port is in.

For now the work around for this would be to set: notify_nova_port_active=False
in neutron to prevent neutron from sending the notification and setting:
vif_plugging_is_fatal=False in nova.conf. Doing this will keep the current
interaction where an instance will be booted without waiting for the network
to be ready.

DocImpact

implements blueprint: nova-event-callback

Change-Id: I4177124485b986706fcf8e73b928024b5d82b822