[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
@geemus @maxlinc Ah, totally up to you on use of suite! I like to think my implementation tries to stay relatively idiomatic and simple while being relatively well maintained, but I don't think it has anywhere near the historical evidence of stability of the other project.

This compliance suite looks awesome. I'll look into it. See brandur/json_schema#13.