cyclades: Update subnets.py to support slac
IPv6 subnets now support a boolean attribute called slac. If a request for a subnet contains both a slac field and an enable_dhcp field, only the slac one will be used. If no slac field is found for an IPv6 subnet, the field enable_dhcp will be used instead. IPv4 subnets silently ignore any slac field if present and only use enable_dhcp. This commit adds the relevant tests in tests/subnets.py too.