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
Add SERVICE_TOKEN to the sample local.conf

I expect the sample local.conf file to contain all the required
variables. However, after copying samples/local.conf, the stack.sh
script still asks for SERVICE_TOKEN.

This commit adds the SERVICE_TOKEN definition to local.conf so that a
new user can just copy samples/local.conf, edit it, and have a running
stack.sh without the mild annoyance of having more variables to set.

Change-Id: Ifb163c7f67c80037d8b10458f2df458a20da4667
Merge "Add DVR support to DevStack"
Merge "Add SERVICE_TOKEN to the sample local.conf"