ACME-DNS: Verify that CNAME for challenge maps correctly

[ericcan]

The following is a feature request.

Current behavior:
In AcmeDnsClient, the user is instructed in the case of a new acme-dns registration to create a CNAME for the _acme-challenge subdomain. In the case of an existing registration, the user is shown a message that says what the mapping should be.

Proposed behavior:
New registration: Once the user confirms they have set up the CNAME record, we could

1. Look up the DNS CNAME record on the _acme-challenge subdomain
2. Report the result of the query
3. If it matches what is registered, confirm that it is correct
4. If it doesn't match, restate what the mapping should be and give user the option to retry (to allow for rechecking DNS records or just to give additional time for the change to propagate) or ignore.

Existing registration: we would eliminate the message saying what the map should be. Instead, we would do a DNS lookup, report the result, and if not correct give an option to retry or ignore, as above.

This test (without the retry/ignore) could be added to unattended mode as well for informational purposes.

[WouterTinus]

Great idea, this can even open up the the acmedns-plugin to unattended operation, at least for pre-configured domains. We can start implementing this after the improved DNS resolver lands (#1067).

[WouterTinus]

The interactive part is done, some support for unattended mode should still be added.

[WouterTinus]

Released in 2.0.5