Add option for nova endpoint type

Add option for nova endpoint type

When the neutron notification to nova was updated to use novaclient the
nova_url parameter was disabled.  This prevents administrators from
using anything but the publicURL as the proper endpoint to notify nova.
This patch adds an option to pass on to novaclient for the
endpoint_type so that the administrator can set the notification url to
public, internal or admin.

Change-Id: I405f761944449cab6b8c8895f98419f79cd74cad
Closes-Bug: #1478471
DocImpact: Need to add a new option to the neutron configuration
reference.