Fix IPv6 Subnet Slaac Check

Fix IPv6 Subnet Slaac Check

Currently, "is_slaac_subnet" method only checks "ipv6_address_mode".
This will let neutron allocate a stateful IPv6 address for instance
when ipv6_ra_mode set to "stateless" or "slaac", and "ipv6_address_mode"
unset. But actually, instance get IPv6 address prefix from radvd, and
auto configure IPv6 address by itself. This change will let "is_slaac_
subnet" also check ipv6_ra_mode attribute, which will no longer allow
fixed IPs on auto-address subnets and delete subnet will correctly delete
auto-address ports.

Change-Id: Ic99193a0a2669fb281759d9b47c7aae6a4f6ecce
Closes-Bug: 1406436