Fixes non-deterministic UT failure around next_auto_assign_ip
Fixes RM6279

Fixes change in place from c59e729744ac8e015089ad583266514de52f94f9

Two different code paths were invoking the ip_address filter, with
different types of variables. One as a netaddr.IPAddress object, the
other with a list of IPv6 integer values. This fix now expects a
list of netaddr.IPAddress objects, creating a list when a single
netaddr.IPAddress object is passed in.