NSX: Fix foreign key constraint delete provider network

NSX: Fix foreign key constraint delete provider network

The cascade delete on the tz_network_bindings table is not
taking effect with the NSX plugin. In order to fix this, this
will require a db migration. This patch allows us to provide a
fix for this by deleting the tz_network_bindings within a transaction
to avoid the foreign key constraint. This patch fixes it this way so
that it can be backported to icehouse. Later a patch will be pushed with a
migration which corrects the db schema.

Closes-bug: #1367032

Change-Id: I19d389acc710224baff79ad114fab756b2e21cfc