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.
Jclouds implements APIs in a declarative way, and automatically generates the request based on the method declaration. For the `create` method, there is [this fallback](https://github.com/jclouds/jclouds/blob/master/apis/openstack-nova/src/main/java/org/jclouds/openstack/nova/v2_0/extensions/FloatingIPAsyncApi.java#L105) defined, which will make the method return `null` when the server returns a 404 response. For responses codes that are not explicitly mapped in the API method, the [error handler](https://github.com/jclouds/jclouds/blob/master/apis/openstack-nova/src/main/java/org/jclouds/openstack/nova/v2_0/handlers/NovaErrorHandler.java#L73) is invoked, so:

* The API will return `null` when the server returns a 404.
* It will throw the IRE when the server returns a 400 with a `quota exceeded` message.

This is related to the test I also mentioned mentioned. It currently covers [the 400 response](https://github.com/jclouds/jclouds/blob/master/apis/openstack-nova/src/test/java/org/jclouds/openstack/nova/v2_0/compute/functions/AllocateAndAddFloatingIpToNodeExpectTest.java#L97-L130) path, but not the 404 one (that's the one I asked to add by copying the existing 400 test).