[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 #2909
@geemus I couldn't see a way to have an Array in the format that was nullable *and* had a format within it. Is that possible?