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.
One option:

```
diff --git a/templates/default/neutron.conf.erb b/templates/default/neutron.conf.erb
index 6b76d28..ae8a078 100644
--- a/templates/default/neutron.conf.erb
+++ b/templates/default/neutron.conf.erb
@@ -12,7 +12,15 @@ ovs_use_veth = <%= @neutron_ovs_use_veth %>
 <% end %>
 <% if @neutron_plugin == "ml2_linuxbridge" %>
 core_plugin = neutron.plugins.ml2.plugin.Ml2Plugin
+  <% if ! @service_plugins.empty? %>
+service_plugins = neutron.services.l3_router.l3_router_plugin.L3RouterPlugin,<%= @service_plugins.join(",") %>
+  <% else %>
 service_plugins = neutron.services.l3_router.l3_router_plugin.L3RouterPlugin
+  <% end %>
+<% else %>
+  <% if ! @service_plugins.empty? %>
+service_plugins = <%= @service_plugins.join(",") %>
+  <% end %>
 <% end %>
 allow_overlapping_ips = <%= @overlapping_ips %>
 use_namespaces = <%= @neutron_namespace %>
@@ -24,10 +32,6 @@ rpc_backend = neutron.openstack.common.rpc.impl_kombu
 notification_driver = <%= @notification_driver %>
 notification_topics = <%= @notification_topics %>
 
-<% if ! @service_plugins.empty? %>
-service_plugins = <%= @service_plugins.join(",") %>
-<% end %>
-
 # keystone auth - this will cascade to all agents/services
 auth_strategy = keystone
 auth_url = <%= @keystone_protocol %>://<%= @keystone_api_ipaddress %>:<%= @keystone_admin_port %><%= @keystone_path %>
```