Fix IPv6 tests if ipv6-private-subnet is stateless

If the cloud has configured the ipv6-private-subnet to use stateless
addressing, such as slaac or dhcpv6-stateless, we need to not request
a fixed IP from neutron.

Story: 2006164
Task: 35674

Change-Id: I27e82b34a39bea8a987724e013081079f236894d
2 files changed