None metadata is not allowed

This fixes recent launch failures with error `Launching server failed: TypeError("'NoneType' object does not support item assignment",)`
I actually responded before you had put the link :). I like the idea of using nova's json schema directly in otter. (We've been wanting this for a while now). There are two options to do that: 1) Copy paste the schema in our code. 2) Have nova as dependent package and use imported object. I like the second one but I am not sure if there any complications in doing that.