port_pair_group_parameters should be possible to update

[Ivar Lazzaro]

As of Today, PPG port_pair_group_parameters is declared with allow_put: False

'port_pair_group_parameters': {
            'allow_post': True, 'allow_put': False,
            'is_visible': True, 'default': None,

Drivers might be able to support the update of such attribute without having to destroy and re-create the whole port chain (and port pair group). We should make this field updateable and have drivers reject the operation whenever not supported.