neon-desktop: IP conflict (ARP problem?)

[jefflill]

@marcusbooyah reported that neon-desktop reported an IP conflict when configuring the desktop cluster.

I suspect that this is an ARP cache problem. We try to identify IP in-use addresses by:

1. Clearing the ARP cache
2. Pinging the IP address
3. If we get a response we know that the address is in use
4. Otherwise, if we see an ARP cache record for the address, it must be in use as well

I suspect we may need to wait a bit after clearing the ARP cache since it's possible that the Windows command doesn't perform this operation synchronously.

[jefflill]

CLOSING: I've disabled the ARP code for now. I think we may be experiencing a race condition here.