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.
If we don't throw the NPE here it gets caught downstream and could be mis-construed as something else whereas here we can force the NPE at the point it pops. If we throw an IRE we are back in the same boat we started in ...