[OVN] Audit the L2/L3 extensions support

[Rodolfo Alonso]

The ML2/OVN mechanism driver has the supported L2 and L3 extensions hardcoded in [1]. It is needed to review the list of supported extensions in ML2/OVN and Neutron and provide:
* A list of not supported extensions.
* For any extension that should be supported and is not in the hardcoded list, provide a fix.

[1]https://github.com/openstack/neutron/blob/c6ac441a5160b79c48d04596ab464e0bac9f6592/neutron/common/ovn/extensions.py

[Miguel Lavalle (minsel)]

@Rodolfo.

What you are saying is that there are extension implementations that should be added to neutron/common/ovn/extensions.py? Or that an analysis is needed to decide what extensions should be implemented for ML2/OVN, implement them and then update neutron/common/ovn/extensions.py?

[Rodolfo Alonso (rodolfo-alonso-hernandez)]

We need to do both:
* First check what extensions exist in Neutron that are not in the ML2/OVN extensions list.
* Check if the missing extensions can be added to the supported ones (a new LP would be needed per extension).

Finally we should document these extensions that won't be added to the ML2/OVN supported ones, and why.