Return GWG IP protocol (version) when no gateway IP - 2.1 branch
Use return_gateway_groups_array() to build correct GWG list

Version for 2.1 branch
Tidy "gateway name - IP" in dropdown list

for 2.1 branch
Tidy up GWG dropdown selection tests

Tested this making a new rule, and editing existing IPv4, IPv6 and IPv4+Ipv6 rules, and switching the IP version on an existing rule. Seems to work!
Merge pull request #991 from phil-davis/RELENG_2_1

Return GWG IP protocol (version) when no gateway IP - 2.1 version