feat(neutron-understack): add L3 router example plugins

These plugins serve the purpose of being able to be loaded and be used
for conversations around different router backend implementations. The
networking design doc references these plugins in its examples.

Author: cardoe

components/neutron/values.yaml

@@ -83,7 +83,12 @@ conf:
       # add 50 to the max MTU we want of 9000 to handle Neutron's -50 for VXLAN type
       global_physnet_mtu: 9050
     service_providers:
-      service_provider: "L3_ROUTER_NAT:cisco-asa:neutron_understack.l3_service_cisco_asa.CiscoAsa"
+      service_provider:
+        type: multistring
+        values:
+          - "L3_ROUTER_NAT:cisco-asa:neutron_understack.l3_router.cisco_asa.CiscoAsa"
+          - "L3_ROUTER_NAT:palo-alto:neutron_understack.l3_router.palo_alto.PaloAlto"
+          - "L3_ROUTER_NAT:vrf:neutron_understack.l3_router.vrf.Vrf"
     ovn:
       # the ovn-metadata-agent utilizes 'localport' on each hypervisor in OVS to work, since
       # we don't have an OVS that the baremetal nodes are plugged into we can't have this

python/neutron-understack/neutron_understack/l3_router/__init__.py

@@ -0,0 +1,5 @@
+from neutron.services.ovn_l3.service_providers.user_defined import UserDefined
+
+
+class CiscoAsa(UserDefined):
+    pass

python/neutron-understack/neutron_understack/l3_router/cisco_asa.py

@@ -0,0 +1,5 @@
+from neutron.services.ovn_l3.service_providers.user_defined import UserDefined
+
+
+class PaloAlto(UserDefined):
+    pass

python/neutron-understack/neutron_understack/l3_router/palo_alto.py

@@ -0,0 +1,5 @@
+from neutron.services.ovn_l3.service_providers.user_defined import UserDefined
+
+
+class Vrf(UserDefined):
+    pass