delete and retry expired authorizations

[aripringle]

If an authorization challenge for a domain has expired, LetsEncrypt returns a response like:

{'status': 404, 'type': u'urn:acme:error:malformed', 'detail': u'Expired authorization'}

In this event, the code will now delete the domain's config file from S3 and retry the authorize_domain call, which will create a new authorization challenge.