Remove locking from network and subnet delete op

Remove locking from network and subnet delete op

delete_subnet in Ml2 plugin instead of using SELECT FOR
UPDATE deletes the IPAllocations that can be auto-deleted
straight away.
An exception is raised if there are ports that cannot be
autodeleted.

delete_network in ML2 plugin tries to delete all ports
and subnets before performing the network deletion.
No lock is needed here - if some other process modifies
the Port or Subnet table, adding new items, the network
deletion will fail because of a violation of a foreign
key contraint.
In that case the operation will be retried.

Change-Id: Ib4e9441a95d6c80b92a43d55fdeb18d7b51a1cf3
Closes-bug: #1332917