(#10274) Nullify addresses with zero prefixlen

(#10274) Nullify addresses with zero prefixlen

Modify the behaviour of Util::Firewall.host_to_ip, as used by the type to
parse source and destination addresses, to return nil if the resulting CIDR
represented address has a prefix length of zero. Includes type and provider
tests for IPv4 and IPv6.

IPtables silently omits rules with source and destination addresses that
have a prefix length of zero (eg. 0.0.0.0/0) because they are functionally
equivialent to not specifying any address. This was causing rules to be
unecessarily reloaded.

The behaviour of Util::IPcidr remains the same. Now includes some additional
tests for it's identification of zero prefixlen IPv4 and IPv6 addresses.