The previous upgrade file created the nsx binding table only in the neutron
lbaas table exists.
This patch created a new upgrade file which creates the table if it doesn't
exist, and adds the foreign key constraints only if the neutron-lbaas table
exists.
This bug appeared in some unittest cases.
Change-Id: Ia23cda570e2012ad2dcdc07fbcc7bcc7629f3c7b