Make flavor id optional via flavor creation API

While there's a great reason to *allow* specifying the id of the flavor you're creating (e.g. you want to "replace" an existing flavor) the more common use case is actually to create a new flavor in which case you don't care what the ID is and *requiring* that the end-user specify it is actually counter-productive, confusing, and potentially frustrating.

Ideally that parameter would be optional and if not specified Nova would simply assign the flavor an unused ID automatically.