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
Merge "Use EUI64 for IPv6 SLAAC when subnet is specified"