Add check for subnet update with conflict gateway and allocation_pools

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