[zuul-core] Remove deprecated port attrs. Use `_ADDRESS` instead.

Merge pull request Netflix#898 from Netflix/revert-ribbon

Downgrade ribbon to fix proxying

Release 2.1.8

Release v2.1.7

Release v2.1.6

Need public static access to the `BaseServerStartup.defaultChannelCon…

…fig()` method for some unit-tests

remove ZuulNettyReqRespTransformer

Add a metric to track unresolved DNS hosts

minor cleanup of filter loading

Merge branch '2.1' of https://github.com/Netflix/zuul into 2.1