Automate OVS bridge creation for multiple mappings

Allows the definition of the global variable OVS_BRIDGE_MAPPINGS (e.g.
in local.conf) to automatically trigger the creation of multiple OVS
bridges. For example:

OVS_BRIDGE_MAPPINGS=physnet1:br-br-enp0s20f1,physnet2:br-enp0s20f2

should automatically yield the creation of two bridges, respectively
associated to the two physical networks declared,
by simply running DevStack with the OVS agent enabled.

Documentation has also been added to doc/source/guides/neutron.rst.

Change-Id: I79dc0213c9d70ba628621c4c0f65481783590085
Closes-Bug: #1535835
diff --git a/lib/neutron_plugins/openvswitch_agent b/lib/neutron_plugins/openvswitch_agent
index b1acacd..94a2689 100644
--- a/lib/neutron_plugins/openvswitch_agent
+++ b/lib/neutron_plugins/openvswitch_agent
@@ -37,6 +37,7 @@
 }
 
 function neutron_plugin_configure_plugin_agent {
+    local mappings_array mapping phys_bridge
     # Setup integration bridge
     _neutron_ovs_base_setup_bridge $OVS_BRIDGE
     _neutron_ovs_base_configure_firewall_driver
@@ -58,9 +59,15 @@
     # complex physical network configurations.
     if [[ "$OVS_BRIDGE_MAPPINGS" == "" ]] && [[ "$PHYSICAL_NETWORK" != "" ]] && [[ "$OVS_PHYSICAL_BRIDGE" != "" ]]; then
         OVS_BRIDGE_MAPPINGS=$PHYSICAL_NETWORK:$OVS_PHYSICAL_BRIDGE
+    fi
 
-        # Configure bridge manually with physical interface as port for multi-node
-        _neutron_ovs_base_add_bridge $OVS_PHYSICAL_BRIDGE
+    if [[ "$OVS_BRIDGE_MAPPINGS" != "" ]]; then
+        IFS=',' read -a mappings_array <<< "$OVS_BRIDGE_MAPPINGS"
+        for mapping in "${mappings_array[@]}"; do
+            phys_bridge=`echo $mapping | cut -f 2 -d ":"`
+            # Configure bridge manually with physical interface as port for multi-node
+            _neutron_ovs_base_add_bridge $phys_bridge
+        done
     fi
     if [[ "$OVS_BRIDGE_MAPPINGS" != "" ]]; then
         iniset /$Q_PLUGIN_CONF_FILE ovs bridge_mappings $OVS_BRIDGE_MAPPINGS