Add some constants to the network model for drivers to use

Following on from

  commit 85f857e62a5edacbf467c19add6e51565ae3669f
  Author: Gary Kotton <gkotton@redhat.com>
  Date:   Sat Dec 8 08:54:16 2012 +0000

    Add vif_type to the VIF model

add some constants to nova.network.model to enumerate the valid
values for 'vif_type'. Also add a constant for max bridge name
length

Blueprint: libvirt-vif-driver
Change-Id: I47949c06fee732681bf0b547577f22a319d4e408
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Make nova network manager fill in vif_type

Make the Nova network manager impl fill in the 'vif_type' parameter
in the VIF configuration for instances. This is hardcoded to
VIF_TYPE_BRIDGE, since that's all Nova network supports.

Blueprint: libvirt-vif-driver
Change-Id: I46d1f0edc820ccc198e5a5c876ea25644ebedd86
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Add some constants to the network model for drivers to use"
Merge "Make nova network manager fill in vif_type"