Validate that L2pop is enabled with DVR

[Dmitrii Shcherbakov]

When overlay networks are used with DVR L2 population is a requirement.

https://docs.openstack.org/neutron/stein/admin/config-dvr-ha-snat.html#network-nodes

https://github.com/openstack/neutron/blob/stable/stein/neutron/plugins/ml2/drivers/openvswitch/agent/ovs_neutron_agent.py#L2358-L2364
    def _check_agent_configurations(self):
        if (self.enable_distributed_routing and self.enable_tunneling and
                not self.l2_pop):

            raise ValueError(_("DVR deployments for VXLAN/GRE/Geneve "
                               "underlays require L2-pop to be enabled, "
                               "in both the Agent and Server side."))

There is a scenario where tunneling is disabled and all networks (tenant and provider) are VLAN networks - only in this case l2 population is not mandatory for DVR. This is not our primary use-case but it clarifies that L2 population cannot be disabled when VXLAN/GRE/GENEVE are used.

The charm should validate this to avoid operator errors.

See also:

https://bugs.launchpad.net/neutron/+bug/1417633

https://review.opendev.org/#/c/165311/ (check that L2 pop is enabled with DVR)
https://review.opendev.org/#/c/166707/ (check that L2 pop is enabled with DVR but only when tunneling is enabled)
https://openstack.nimeyo.com/42337/openstack-dev-dvr-and-l2-population