Revert "Change Neutron's default ML2 driver to OVN"

This reverts commit 5c304d817682d6c807b532b50a2f105479ac3fa2.

Reason for revert: There are more things to fix/move like done in 791085 and 791282 Also let's change all required default in devstack scripts instead of devstack's zuul job side. Basically do this change without any change in .zuul.yaml

Change-Id: Ie0f59d1b9a4b97ad9fd8131819054dfb616f31fd
diff --git a/.zuul.yaml b/.zuul.yaml
index e133bfa..00129b5 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -468,10 +468,6 @@
         SWIFT_HASH: 1234123412341234
         DEBUG_LIBVIRT_COREDUMPS: true
         NOVA_VNC_ENABLED: true
-        OVN_L3_CREATE_PUBLIC_NETWORK: true
-        OVN_DBS_LOG_LEVEL: dbg
-        ENABLE_CHASSIS_AS_GW: true
-        Q_USE_PROVIDERNET_FOR_PUBLIC: true
       devstack_local_conf:
         post-config:
           $NEUTRON_CONF:
@@ -481,11 +477,7 @@
         # Core services enabled for this branch.
         # This list replaces the test-matrix.
         # Shared services
-        #
-        # NOTE(lucasagomes): disable dstat until bug
-        # https://bugs.launchpad.net/ubuntu/+source/dstat/+bug/1866619 is
-        # fixed. Also see: https://bugs.launchpad.net/neutron/+bug/1898863
-        dstat: false
+        dstat: true
         etcd3: true
         memory_tracker: true
         mysql: true
@@ -504,14 +496,13 @@
         n-sch: true
         # Placement service
         placement-api: true
-        # OVN services
-        ovn-controller: true
-        ovn-northd: true
-        ovs-vswitchd: true
-        ovsdb-server: true
         # Neutron services
+        q-agt: true
+        q-dhcp: true
+        q-l3: true
+        q-meta: true
+        q-metering: true
         q-svc: true
-        q-ovn-metadata-agent: true
         # Swift services
         s-account: true
         s-container: true
@@ -536,24 +527,15 @@
           # Core services enabled for this branch.
           # This list replaces the test-matrix.
           # Shared services
-          #
-          # NOTE(lucasagomes): disable dstat until bug
-          # https://bugs.launchpad.net/ubuntu/+source/dstat/+bug/1866619 is
-          # fixed. Also see: https://bugs.launchpad.net/neutron/+bug/1898863
-          dstat: false
+          dstat: true
           memory_tracker: true
           tls-proxy: true
           # Nova services
           n-cpu: true
           # Placement services
           placement-client: true
-          # OVN services
-          ovn-controller: true
-          ovn-northd: false
-          ovs-vswitchd: true
-          ovsdb-server: true
           # Neutron services
-          q-ovn-metadata-agent: true
+          q-agt: true
           # Cinder services
           c-bak: true
           c-vol: true
@@ -567,7 +549,6 @@
           # integrated gate, so specifying the services has not effect.
           # ceilometer-*: false
         devstack_localrc:
-          ENABLE_CHASSIS_AS_GW: false
           # Subnode specific settings
           GLANCE_HOSTPORT: "{{ hostvars['controller']['nodepool']['private_ipv4'] }}:9292"
           Q_HOST: "{{ hostvars['controller']['nodepool']['private_ipv4'] }}"
diff --git a/lib/neutron_plugins/ml2 b/lib/neutron_plugins/ml2
index a58ba5c..ae4b251 100644
--- a/lib/neutron_plugins/ml2
+++ b/lib/neutron_plugins/ml2
@@ -8,14 +8,14 @@
 set +o xtrace
 
 # Default openvswitch L2 agent
-Q_AGENT=${Q_AGENT:-ovn}
+Q_AGENT=${Q_AGENT:-openvswitch}
 if [ -f $TOP_DIR/lib/neutron_plugins/${Q_AGENT}_agent ]; then
     source $TOP_DIR/lib/neutron_plugins/${Q_AGENT}_agent
 fi
 
 # Enable this to simply and quickly enable tunneling with ML2.
 # Select either 'gre', 'vxlan', or 'gre,vxlan'
-Q_ML2_TENANT_NETWORK_TYPE=${Q_ML2_TENANT_NETWORK_TYPE:-"geneve"}
+Q_ML2_TENANT_NETWORK_TYPE=${Q_ML2_TENANT_NETWORK_TYPE:-"vxlan"}
 # This has to be set here since the agent will set this in the config file
 if [[ "$Q_ML2_TENANT_NETWORK_TYPE" == "gre" || "$Q_ML2_TENANT_NETWORK_TYPE" == "vxlan" ]]; then
     Q_TUNNEL_TYPES=$Q_ML2_TENANT_NETWORK_TYPE
@@ -24,7 +24,7 @@
 fi
 
 # List of MechanismDrivers to load
-Q_ML2_PLUGIN_MECHANISM_DRIVERS=${Q_ML2_PLUGIN_MECHANISM_DRIVERS:-ovn}
+Q_ML2_PLUGIN_MECHANISM_DRIVERS=${Q_ML2_PLUGIN_MECHANISM_DRIVERS:-openvswitch,linuxbridge}
 # Default GRE TypeDriver options
 Q_ML2_PLUGIN_GRE_TYPE_OPTIONS=${Q_ML2_PLUGIN_GRE_TYPE_OPTIONS:-tunnel_id_ranges=$TENANT_TUNNEL_RANGES}
 # Default VXLAN TypeDriver options