Make it possible to enable native neutron PXE.

This is used to control the dhcp_options_enabled flag in nova.conf,
needed to enable PXE orchestration (until Nova assumes all Neutron
libraries and servers support it).

Change-Id: I035a2341d829a9987eeeebdd9c22f039e69489db
Merge "Make it possible to enable native neutron PXE."