When a subnet is created and allocated, the gateway IP is created
based on the subnet CIDR. In case of IPv6 prefix delegation, this
CIDR is a temporary one. In this case the gateway IP cannot be
assigned yet and the value stored in the DB should be "None".
IpamBackendMixin._gateway_ip_str must read properly the IP version
stored in the "subnet" variable, under the key "ip_version"
instead of "version".
Closes-Bug: #1856726
Closes-Bug: #1856675
Change-Id: I9313c880cc458f08dc3a1b0ff13187b764ba7042