Use Field in fixed_ip

In fixed_ip object, we use IPV4AndV6Address, which is a FieldType
instead of Field.

Create a Field for IPV4AndV6Address and use it in the fixed_ip object.

Change-Id: I0f369a70f755bc7e290edb96656c7c0cb2f3a00a
Merge "Use Field in fixed_ip"