Fixes #158

* Refactored IPPolicy.get_ip_policy_rule_set to use IPRange instead of
  IPNetwork index splicing because index splicing is not supported in netaddr
  for IPv6 networks.

* Added unit test case for IPv6 subnet for
  IPPolicy.get_ip_policy_rule_set

* Modified an IPAM unit test to have correct parameters for test's
  original intent.