MEDIUM: acme: new 'challenge-ready' option

MEDIUM: acme: new 'challenge-ready' option

The previous patch implemented the 'dns-check' option. This one replaces
it by a more generic  'challenge-ready' option, which allows the user to
chose the condition to validate the readiness of a challenge. It could
be 'cli', 'dns' or both.

When in dns-01 mode it's by default to 'cli' so the external tool used to
configure the TXT record can validate itself. If the tool does not
validate the TXT record, you can use 'cli,dns' so a DNS check would be
done after the CLI validated with 'challenge_ready'.

For an automated validation of the challenge, it should be set to 'dns',
this would check that the TXT record is right by itself.