Alembic migration script for Loadbalancing service

Creates tables for:
 * VIPs
 * LB pools
 * Health monitors
 * Members
 * Pool statistics
 * Session persistences
 * Pool-monitor Associations

Change-Id: I96c497a319129eb8a3ce6a1420a38e8efb666b6b
Fixes: bug #1122117
Add check for subnet update with conflict gateway and allocation_pools

Fixes: bug 1062061

The patch will raise exception 'GatewayConflictWithAllocationPools' when
subnet update with conflict gateway and allocation_pools.

Because before validate gateway ip with conflict allocation pools, we need
validate allocation pools first. Move the validation of allocation pools
into _validate_subnet. Then_allocate_pools_for_subnet is only responsible
for pools allocation, and_validate_subnet is responsible for most validate
for subnet.

Change-Id: I8dffe45ce5c02e8e6c317bad470054564538bcf2
Merge "Add check for subnet update with conflict gateway and allocation_pools"