support for nova notifications

Recent versions of Nova and Neutron communicate via an event
notification mechanism such that Nova is aware of the completion of
network interface plugging/unplugging performed by Neutron.

This patch adds the appropriate configuration options to the
puppet-neutron module so that they can be set by client code.

This requires neutron with https://review.openstack.org/#/c/85492/6 or
similar.

Change-Id: I87487ab5dd8eaca10ea5776ac289bf45c18b1a1d
Closes-bug: 1306694