JCLOUDS-607: ComputeService.createNodesInGroup throws NPE on FloatingIPA...

Addresses possible returned null value from 'FloatingIPApi.create()'. If upon check we find null then throw InsufficientResourcesException. This in turn will make a second check at IP allocation by iterating through 'FloatingIPApi.list()' and grabbing an available IP. If this subsequent call fails then re-throw InsufficientResourcesException.
In both cases, when falling back to the list because of the `null` or the IRE, I'd end up throwing an IRE if no ip could be allocated. I assume the NPE was an issue and that it should never got propagated, so I'd throw the IRE if no ip can be allocated, regardless of the cause (404 or 400).