Feature Request Overview (mandatory - Complete while in New status){_}
{}What user goal or problem do you need to solve?{_} 
Current Neutron create a security group with stateful as default.
However, we confirmed that stateful security group introduce poor performance.
If OVS-DPDK enabled deployment, the stateful security group decrease down to 30% than non-SG applied performance.
Due to that, we introduce stateless security group in ML2/OVN in OSP17.1.
However, there is no way to make stateless security group by default.
That means, cloud operator or tenant user needs to pay attention to what type of security group they use to avoid performance issue.
This kind of operation is annoying. 

Business justification (mandatory - Complete while in New status)
How would this feature benefit the customer?
We introduce stateless security group with ML2/OVN in OSP17.1.
Network performance sensitive workload hosted deployments, e.g. telco workload or else, needs this feature to reduce the operational costs.
 

Functional requirements (mandatory - Complete while in New status){_}
{}What do you want the result of this feature to be? Add as many requirements as needed.{_}

•  Make security group as sateless as default by neutron configuration.
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