get_port_groups should be called get_sg_port_groups

[Flavio Fernandes]

OVN's API called get_port_groups is poorly named and has misleading docstring.

It returns only the OVN port groups that map to the security group in Neutron.
Therefore, it should be called get_sg_port_groups.

neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/api.py

    @abc.abstractmethod
    def get_port_groups(self):
        """Gets all port groups in the OVN_Northbound DB