order of attributes in YANG model could use some tweaking

[mcr]

> 25) "last-renewal-date": Some questions regarding the underlying data
> model:

> 25a) would it make sense to put the last-renewal-date next to the
> created-on?

> 25b) It seems that "last-renewal-date" can be present only if there is
> also an "expires-on".  Again, you could simply move this into a
> corresponding choice, e.g. something like: choice validity {   case
> perpetual: {leaf nonce {....}}    case limited: {leaf expires on {...}
>                 {leaf last-renewal-date{...}}

> (and, if within the choice any node must be present, indicate by making
> the corresponding leaf mandatory within the choice, not just the choice
> itself - not sure if this applies here)