Add AFTER_SPAWN event to callbacks
Add AFTER_SPAWN event to callbacks

This adds new callback event AFTER_SPAWN so that follows a change[1]
has been added to neutron tree.

[1]: https://review.openstack.org/#/c/462536/

Change-Id: I50513229b207c6a0a0b0b0a5e00873fb634f6b67

openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib,openstack/neutron-lib