LBaaS V2 listener delete leaves SNI container behind

[German Eichberger]

Looking att the database code when a listener is created with SNI support the respective entry is added to the SNI table but when the listener is deleted the SNI entry is left behind. There is no special delete call for SNI so listener should clean that up.

[ugvddm (271025598-9)]

In defination of class Listener, we can find that:
sni_containers = orm.relationship(
            SNI,
            backref=orm.backref("listener", uselist=False),
            uselist=True,
            lazy="joined",
            primaryjoin="Listener.id==SNI.listener_id",
            order_by='SNI.position',
            collection_class=orderinglist.ordering_list(
                'position'),
            foreign_keys=[SNI.listener_id],
            cascade="all, delete-orphan"
    )
Listener have configured delete/delete-orphan Cascade for sni_containers, when the listener be deleted, as well as sni be deleted automatically, please refer to:
http://docs.sqlalchemy.org/en/rel_1_0/orm/tutorial.html#configuring-delete-delete-orphan-cascade