Add DVR support to lib/neutron
Add DVR support to lib/neutron

Added NEUTRON_DISTRIBUTED_ROUTING to more easily control DVR
configuration.  If set to True, DVR will be enabled and the
default agent mode will be set to 'dvr_snat' since that works
with all types of routers by default.  Advanced users can
override that by setting NEUTRON_DVR_MODE, for example in
multi-node configurations where different agent modes are
desired.

This should bring lib/neutron inline with lib/neutron-legacy
in supporting all the different DVR modes.

Change-Id: I9f25921eefc5b935aad3bb1edc5e41ee0ce43a84

dtroyer/devstack,group-policy/devstack,nawawi/openstack,nawawi/openstack,openstack-dev/devstack,noironetworks/devstack,noironetworks/devstack,dtroyer/devstack,group-policy/devstack,openstack-dev/devstack