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/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