Add Neutron DVR configuration variables

This change modifies the l3_agent.ini, neutron.conf and ml2_conf.ini image
files to add parameters necessary for configuring Neutron distributed
virtual routers.

Change-Id: I8b845e2436465d355183e919f741726c9593b177
blueprint: support-neutron-dvr
Merge "Add Neutron DVR configuration variables"