Add DVR support to DevStack

This patch introduces a new Neutron  config option: Q_DVR_MODE.

The possible values are:

- legacy  : No DVR functionality
- dvr_snat: Controller/single node DVR
- dvr     : Compute node in multi-node DVR

When DVR is enabled the following configuration changes will be
in place:

- neutron.conf
  - router_distributed=True

- l3_agent.ini
  - agent_mode=$Q_DVR_MODE

- ovs_neutron_plugin.ini:
  - enable_distributed_routing=True
  - l2_population=True
  - tunnel_types=vxlan

Supports-blueprint: neutron-ovs-dvr

Change-Id: I669e054a8489fe7ac7f5c00df6535349d477e8c4
Merge "Add DVR support to DevStack"