add availability_zone to l3 agent and dhcp agent configs

[Dmitrii Shcherbakov]

Currently charm-neutron-gateway does not render the availability zone config property in either dhcp or l3 agent config section:

[AGENT]
availability_zone = zone-1

https://docs.openstack.org/neutron/latest/admin/config-az.html#availability-zone-of-agents
"The availability_zone attribute can be defined in dhcp-agent and l3-agent. To define an availability zone for each agent, set the value into [AGENT] section of /etc/neutron/dhcp_agent.ini or /etc/neutron/l3_agent.ini:"

The dhcp agent part is only done for charm-neutron-openvswitch currently with AZ values taken from the principal (nova-compute) charm.

https://docs.openstack.org/neutron/latest/admin/config-az.html#achieving-high-availability-with-availability-zone
"Although, the Networking service provides high availability for routers and high availability and fault tolerance for networks’ DHCP services, availability zones provide an extra layer of protection by segmenting a Networking service deployment in isolated failure domains. By deploying HA nodes across different availability zones, it is guaranteed that network services remain available in face of zone-wide failures that affect the deployment."

Network availability zone is a separate extension:

https://docs.openstack.org/neutron/latest/admin/config-az.html#required-extensions

network_availability_zone and availability_zone are ML2 core plugin extensions:
https://github.com/openstack/neutron/blob/stable/queens/neutron/plugins/ml2/plugin.py#L154-L155

router_availability_zone is the L3 service extension:
https://github.com/openstack/neutron/blob/stable/queens/neutron/services/l3_router/l3_router_plugin.py#L83-L88

They do not need to be enabled in a special way by the looks of it:

openstack extension list --network | grep -i availa
| Availability Zone | availability_zone | The availability zone extension. |
| Network Availability Zone | network_availability_zone | Availability zone support for network. |
| Network IP Availability | network-ip-availability | Provides IP availability data for each network and subnet. |
| Router Availability Zone | router_availability_zone | Availability zone support for router.

[Edward Hope-Morley (hopem)]

Looks like this was fixed in the patches submitted to https://bugs.launchpad.net/charm-neutron-gateway/+bug/1796068