occasional DNS lookup failures for authserver; should retry

[Colin Watson]

We see occasional failures like this, a few a day:

  2016-12-12 12:15:38+0000 [-] [Failure instance: Traceback (failure with no frames): <class 'twisted.internet.error.DNSLookupError'>: DNS lookup failed: address 'xmlrpc.lp.internal' not found: [Errno -2] Name or service not known.

This is obviously a network-level problem, but I think it would be a good idea if we had some kind of retry logic in place for talking to the authserver before it gets to the point of showing up as a user-visible failure.