ml2/ovn should not be calling neutron.api.rpc.agentnotifiers.dhcp_rpc_agent_api

While using a ml2/ovn deployment, there is no need for using RPC based DHCP, since that is
handled via ovn-controller. Still, we currently enable DhcpAgentNotifyAPI unconditionally [0],
which seems wrong.

One side effect of that is that we periodically see the following warning log. Good news is that
this does not seem to be breaking anything.

2021-03-25 20:23:59.155 28 WARNING neutron.scheduler.dhcp_agent_scheduler [req-d28046f6-897a-4644-a64e-8338b81d0676 72126141799d44f6a82b803bde3ed144 b3d574eefb1b4eba88ba2b93a2b3cd21 - default default] No more DHCP agents

2021-03-25 20:23:59.158 28 WARNING neutron.api.rpc.agentnotifiers.dhcp_rpc_agent_api [req-d28046f6-897a-4644-a64e-8338b81d0676 72126141799d44f6a82b803bde3ed144 b3d574eefb1b4eba88ba2b93a2b3cd21 - default default] Unable to schedule network c578ba7a-156a-4ec5-b7c9-37aeba1331d3: no agents available; will retry on subsequent port and subnet creation events.

According to Rodolfo, we must be careful in adding a conditional to that code, so it does not adversely affect nova.

He also points out that there is a flag that may be what we can use for this.
"""
<ralonsoh> so, IMO, we should make this conditional
<ralonsoh> (or maybe that's more complicated, we need to know that no compute is running a DHCP agent)
<ralonsoh> but be careful: we cannot disable this if we don't know any compute is not using dhcp agents
<ralonsoh> there is a flag (in master) cfg.CONF.enable_traditional_dhcp
<ralonsoh> used to enable/disable this RPC
<ralonsoh> related to the DHCP server on OVS (a new feature not merged yet)
<ralonsoh> we can implement something similar
"""

[0]: https://github.com/openstack/neutron/blob/de3681ed3b0d0a54f0b8b59628fa50f25d5e737e/neutron/plugins/ml2/plugin.py#L325-L327