Use EUI64 for IPv6 SLAAC when subnet is specified

Use EUI64 for IPv6 SLAAC when subnet is specified

This commit uses EUI64 for SLAAC and stateless IPv6 address
when subnet id in fixed_ip is specified.

After this patch, all the ports created on a subnet which has
ipv6_address_mod=slaac or ipv6_address_mod=dhcpv6-stateless
will use EUI64 as the address.
This patch also checks if fixed IP address is specified
for a IPv6 subnet with address mode slaac or dhcpv6-stateless
during creating or updating a port. If yes, raise InvalidInput
error to stop the port creation or update.

Remove unit test test_generated_duplicate_ip_ipv6 because
fixed_ip should not be specified for a slaac subnet.

Change-Id: Ie481cfb2f4313baf44bf1a838ebda374a5c74c6a
Closes-Bug: 1330826