[rackspace] More optional parameters for Cloud Load Balancers

This makes the `:nodes` and `:port` parameters optional for `create_load_balancer()` and `load_balancers.create()`. The docs (and testing against the API) indicate that `:virtual_ips` *is* required.

/cc @elight, @maxlinc

fixes #2902
Thanks for the note about brandur/json_schema. I think I looked at it a long time ago and got the opposite impression at that time. I'll take another look.

FYI - there's now a [JSON-Schema compliance suite](https://github.com/json-schema/JSON-Schema-Test-Suite#who-uses-the-test-suite) that's used by used by a lot of implementations it's not used by either Ruby implementor... yet.