Code review comment for ~dojordan/cloud-init:azure-preprovisioning

Sorry about that, totally missed this one. I agree, I think we should pin the context manager to the nic. When we get an exception, I think it is reasonable to look for the fallback nic again. I'll make that change now.

> I'd like a response to a comment in my last review:
> | without passing in a nic to EphemeralDHCPv4, it will end up calling
> | 'find_fallback_nic' each time. I wonder if that behavior is better or
> | worse than picking one device and just sticking with it throuh the use
> | of this class.
>
> I dont expect that this is a big deal, but a hotplug of a nic during
> the wait for the IMDS could result in the interface that is used for
> dhcp to be different on retries than on the first.