Add use_file_injection key to nova config file

use_file_injection defaults to True in Nova and we need
to be able to override this to use neutron-dhcp-agent.

Co-Authored-By: Derek Higgins <derekh@redhat.com>

Partial-Bug: #1241042

Change-Id: Id2732efa434db9578bd8d5fcfe9a51bea2824db6