Ensure non-overlapping cidrs in subnetpools with galera

Ensure non-overlapping cidrs in subnetpools with galera

This change enables galera support in _lock_subnetpool[1]. It uses an
update to disallow 2 transactions performing concurrent subnet
allocation in the same subnetpool to succeed: the 2 transactions will
conflict because they update the same row so the db (including Galera
multi-writer cluster) will discard the last transaction and
Controller.create[2] will catch and retry the "discarded" allocation.

This change adds the "hash" attribute in "subnetpools" table to enable
previous update.

[1] neutron.ipam.subnet_alloc.SubnetAllocator
[2] neutron.api.v2.base

Change-Id: I74f7100a6fd9b7787be693adffec15ec468d0018
Closes-Bug: #1451576