Added LinuxBridge support (vlan tenant networks only, no vxlan support)

Added new plugin "ml2_linuxbridge" to install and lay down ML2/LinuxBridge config files. No support for vxlan, as that requires newer kernel and iproute2. 
This is meant for new installs ONLY. Uses a new DB: neutron_ml2. Do not switch from OVS to ML2/LB; unexpected results will occur.
This needs to be merged with service_plugins below:
```<% if ! @service_plugins.empty? %>
service_plugins = <%= @service_plugins.join(",") %>
<% end %>```

Otherwise, if you have an override for LoadBalancerPlugin for example, you get:
```
# fgrep -n service_plugins /etc/neutron/neutron.conf 
10:service_plugins = neutron.services.l3_router.l3_router_plugin.L3RouterPlugin
22:service_plugins = neutron.services.loadbalancer.plugin.LoadBalancerPlugin
```

2014-09-23 01:51:32.414 15604 WARNING neutron.api.extensions [-] Extension router not supported by any of loaded plugins

2014-09-23 02:17:37.674 38030 DEBUG neutron.service [-] service_plugins                = ['neutron.services.loadbalancer.plugin.LoadBalancerPlugin'] log_opt_values /usr/lib/python2.7/dist-packages/oslo/config/cfg.py:1941

This took a long time to figure out why I had no router endpoint :(