Adds support for Floating IP

I'm fine with addressing it in all places in another PR (that's why I suggested to open an issue to track this).

The change is not about being consistent within the OpenStack APIs or within the rest of the jclouds providers. It's about being aligned with the rest of the world. Users will expect a FileNotFoundException when working with a file that does not exist, a SQLException when working with a table that does not exist, and so on. But a silently returned null in such cases does not make sense.

Regarding the complexity and impact, I'm pretty sure both are little. I've also changed that for the GCE provider and the change is straightforward. The impact should also be low: updating something means that you have had to previously get a reference to that object (when it still exists). So it's very unlikely that someone calls the update method without having done that. The only chance a 404 could happen is if the resource was deleted between the get/search calls and the update one, which I'm pretty sure it's unlikely to happen. The change won't modify the method signature so client code would still compile, and I'm pretty sure the "semantic" change to an exception will affect someone, given the nature of an update workflow. The bug in GCE was different: an invalid (hardcoded) region was used, so an exceprion un this case also makes more sense than a null return value.

Also returning null will force users to introduce a lot of boilerplate code. An exception aborts execution immediately, and there is no need to try/catch all those cases where a failure is unlikely to happen (mostly caused by a client mistake). When using null, we're forcing a null check after every call, otherwise the code execution could continue in a potential inconsistent state, delaying (and possibly masquerading) the real failure.

So, I'm OK with addressing this in another PR, but I think it's something that should be done, for all the mentioned reasons, and I think we can move forward with the change given the low impact it may have.