Prevent update alloc pool over existing gateway ip

Prevent update alloc pool over existing gateway ip

The gateway IP for a subnet is not allowed to be listed in the
allocation pool for that subnet. This restriction is checked and
enforced at subnet-create time.

During subnet-update, it is only partially checked. An
exception is returned if the update request tries to place the gateway
IP in an existing allocation pool OR if both gateway and allocation
pool are being changed and the gateway is located in the new pool.

If only the allocation pool is being updated, no check is made to
verify that the new allocation pool does not contain the existing
gateway IP.

Closes-Bug: #1479514
Change-Id: Id9583d6ad88188955388931cd688ca19bd2c9717