[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
Yeah, I think maybe it has been discussed briefly, but I've taken to using lots of json-schema stuff elsewhere (ie in github.com/interagent stuff) and am a big fan. Would definitely be up for something in this space, although perhaps using https://github.com/brandur/json_schema instead of json-schema (I think it is more supported/up to date).