[OVN] Add support for WSGI mod

[Lucas Alvares Gomes]

Currently the OVN driver initialization does not work with uWSGI because the initialization is different than eventlet and the post_fork_initialization() method [0] does not get called in the process.

The post_fork_initialization() method is the one responsible for a few things:

* Starting the OVN database connections
* Starting the maintenance task
* Overwriting the agent and availability zone methods for OVN use [1]
* Running the database synchronization script (in log or repair mode) [2]

These steps would need to be factored out to adhere to the uWSGI initialization process.

[0] https://github.com/openstack/neutron/blob/19de0598227e2156b26f6d5eca874eaba09106a7/neutron/plugins/ml2/drivers/ovn/mech_driver/mech_driver.py#L181-L186

[1] https://github.com/openstack/neutron/blob/19de0598227e2156b26f6d5eca874eaba09106a7/neutron/plugins/ml2/drivers/ovn/mech_driver/mech_driver.py#L284-L292

[2] https://github.com/openstack/neutron/blob/19de0598227e2156b26f6d5eca874eaba09106a7/neutron/plugins/ml2/drivers/ovn/mech_driver/mech_driver.py#L300-L315

How to reproduce:

Deploy Devstack with OVN (https://docs.openstack.org/neutron/latest/contributor/testing/ovn_devstack.html) with NEUTRON_DEPLOY_MOD_WSGI=True set in local.conf.

[Slawek Kaplonski (slaweq)]

I don't think this is an rfe which we need to discuss in the drivers team and approve. For me it is just yet another feature parity gap which we should close ASAP. So I will remove "rfe" tag from that and lets keep it as a bug.

[Rabi Mishra (rabi)]

Any plans for fixing this soon? Looks like this would need some major refactoring?