Show field defaults if there is no value

It turns out that the default value for a WTForms field is only displayed if the field is not in the cfg dict that we pass to the form at all, not even an empty string or None, so we pop of any fields that are empty anyway.

The only thing is, now we are calling host_memory_usage a few times (3) within this request, it isn't so bad because this particular thing is fast, but a basic cache or @reify property could be nice for this sort of thing later, for those functions that are called a few times in a request.