DHCPv6 network tests

With upcoming IPv6 support we need to test specific
DHCPv6 features that don't exist on IPv4, like
SLAAC, stateless, etc. RFC: 3315, etc.
Partially implements: blueprint ipv6-api-testing-parity

Change-Id: I0a063e543e320ea625a5411547bce7fa2ad66b7d
Merge "DHCPv6 network tests"