Add "radvd_user" config parameter to Neutron L3 container

The username passed to radvd, used to drop root privileges and change
user ID to username and group ID to the primary group of username. If no
user specified, the user executing the L3 agent will be passed. If "root"
specified, because radvd is spawned as root, no "username" parameter
will be passed

Depends-On: https://review.opendev.org/#/c/691846/
Change-Id: Ie9f16f3117e6531f4ef1e4973e61c72dbf904b8a
Closes-Bug: #1844688

deployment/neutron/neutron-l3-container-puppet.yaml

@@ -120,6 +120,16 @@ parameters:
         Comma-separated list of extensions enabled for the Neutron L3 agent.
     type: comma_delimited_list
 
+  NeutronL3AgentRadvdUser:
+    default: "root"
+    description: |
+      The username passed to radvd, used to drop root privileges and change
+      user ID to username and group ID to the primary group of username. If no
+      user specified, the user executing the L3 agent will be passed. If "root"
+      specified, because radvd is spawned as root, no "username" parameter
+      will be passed
+    type: string
+
 conditions:
   keepalived_wrapper_enabled: {equals: [{get_param: NeutronEnableKeepalivedWrapper}, true]}
   haproxy_wrapper_enabled: {equals: [{get_param: NeutronEnableHaproxyDockerWrapper}, true]}
@@ -212,6 +222,7 @@ outputs:
             - l3_agent_extensions_empty
             - {}
             - neutron::agents::l3::extensions: {get_param: NeutronL3AgentExtensions}
+          - neutron::agents::l3::radvd_user: {get_param: NeutronL3AgentRadvdUser}
       service_config_settings:
         map_merge:
           - get_attr: [NeutronBase, role_data, service_config_settings]

releasenotes/notes/l3_agent_radvd_user-1814df18745101d6.yaml

@@ -0,0 +1,6 @@
+---
+other:
+  - |
+    Add "radvd_user" configuration parameter to the Neutron L3 container.
+    This parameter defines the user pased to radvd. The default value is
+    "root".