Set agent modes for DVR+HA multinode job

We created experimental Neutron DVR+HA multinode job with [1].
DVR+HA needs a three node setup with two dvr_snat nodes
and one dvr node. So in this setup, we set
1) controller node's agent_mode to dvr_snat
2) first compute to dvr and other to dvr_snat

[1] https://review.openstack.org/#/c/383742/

Change-Id: I4e08a97158dc4538c6f021c049c6e60fb7293459

devstack-vm-gate.sh

@@ -323,12 +323,12 @@ function setup_localrc {
     fi
 
     if [[ "$DEVSTACK_GATE_NEUTRON_DVR" -eq "1" ]]; then
-        if [[ "$DEVSTACK_GATE_TOPOLOGY" != "aio" ]] && [[ $role = sub ]]; then
-            # The role for L3 agents running on compute nodes is 'dvr'
-            localrc_set $localrc_file "Q_DVR_MODE" "dvr"
-        else
-            # The role for L3 agents running on controller nodes is 'dvr_snat'
+        # The role for L3 agents running on first node is 'dvr' and
+        # other nodes is 'dvr_snat'
+        if [[ "$DEVSTACK_GATE_TOPOLOGY" == "aio" ]] || [[ $role = sub ]]; then
             localrc_set $localrc_file "Q_DVR_MODE" "dvr_snat"
+        else
+            localrc_set $localrc_file "Q_DVR_MODE" "dvr"
         fi
     fi
 

multinode_setup_info.txt

@@ -142,3 +142,8 @@ This is what it all looks like after you run devstack and boot some nodes.
 |   +--------vxlan-tunnel-----------+--------vxlan-tunnel----------+                     |
 |                          |   |                          |   |                          |
 +--------------------------+   +--------------------------+   +--------------------------+
+
+When DVR is enabled, agent_mode in l3_agent.ini for the primary node will be set to "dvr"
+and "dvr_snat" for any remaining subnodes. DVR HA jobs need 3 node setup with this
+configuration, where "dvr_snat" represents the network node with centralized SNAT,
+and "dvr" represents compute nodes. There should be at least 2 "dvr_snat" nodes.